Revert "[infra] Switch back to Xenial on some bots"

This reverts commit 6596e8c6.

Reason for revert: Main bug was fixed.

Original change's description:
> [infra] Switch back to Xenial on some bots
>
> This tests the hypothesis that the current timeout problems are on
> Bionic bots only.
>
> Bug: v8:11818
> Change-Id: I68f84cda52ca392fbda5a400eb2bf136b7ee85a3
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2916816
> Auto-Submit: Michael Achenbach <machenbach@chromium.org>
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#74747}

Bug: v8:11818
Change-Id: Ib5f952dc6f23f3a98bb1d79ae5ce689e288d6727
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2940897
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74978}

infra/testing/builders.pyl

@@ -314,7 +314,7 @@
   # Linux64
   'v8_linux64_asan_rel_ng_triggered': {
     'swarming_dimensions' : {
-      'os': 'Ubuntu-16.04',
+      'os': 'Ubuntu-18.04',
     },
     'tests': [
       {'name': 'test262', 'shards': 7},
@@ -353,13 +353,7 @@
       {'name': 'v8testing', 'variant': 'minor_mc'},
       {'name': 'v8testing', 'variant': 'no_lfa'},
       {'name': 'v8testing', 'variant': 'stress_instruction_scheduling'},
-      {
-        'name': 'v8testing',
-        'variant': 'stress_concurrent_allocation',
-        'swarming_dimensions' : {
-          'os': 'Ubuntu-16.04',
-        },
-      },
+      {'name': 'v8testing', 'variant': 'stress_concurrent_allocation'},
       {'name': 'v8testing', 'variant': 'stress_concurrent_inlining'},
     ],
   },
@@ -507,7 +501,7 @@
   },
   'v8_linux64_tsan_rel_ng_triggered': {
     'swarming_dimensions' : {
-      'os': 'Ubuntu-16.04',
+      'os': 'Ubuntu-18.04',
     },
     'tests': [
       {'name': 'benchmarks'},
@@ -516,18 +510,12 @@
       {'name': 'v8testing', 'shards': 5},
       {'name': 'v8testing', 'variant': 'extra', 'shards': 3},
       {'name': 'v8testing', 'variant': 'slow_path'},
-      {
-        'name': 'v8testing',
-        'variant': 'stress_concurrent_allocation',
-        'swarming_dimensions' : {
-          'os': 'Ubuntu-16.04',
-        },
-      },
+      {'name': 'v8testing', 'variant': 'stress_concurrent_allocation'},
     ],
   },
   'v8_linux64_tsan_no_cm_rel_ng_triggered': {
     'swarming_dimensions' : {
-      'os': 'Ubuntu-16.04',
+      'os': 'Ubuntu-18.04',
     },
     'tests': [
       {
@@ -545,7 +533,7 @@
   },
   'v8_linux64_tsan_isolates_rel_ng_triggered': {
     'swarming_dimensions' : {
-      'os': 'Ubuntu-16.04',
+      'os': 'Ubuntu-18.04',
     },
     'tests': [
       {'name': 'v8testing', 'test_args': ['--isolates'], 'shards': 7},
@@ -1187,13 +1175,7 @@
       {'name': 'v8testing', 'variant': 'no_lfa'},
       {'name': 'v8testing', 'variant': 'slow_path'},
       {'name': 'v8testing', 'variant': 'stress_instruction_scheduling'},
-      {
-        'name': 'v8testing',
-        'variant': 'stress_concurrent_allocation',
-        'swarming_dimensions' : {
-          'os': 'Ubuntu-16.04',
-        },
-      },
+      {'name': 'v8testing', 'variant': 'stress_concurrent_allocation'},
       {'name': 'v8testing', 'variant': 'stress_concurrent_inlining'},
       # Noavx.
       {
@@ -1342,7 +1324,7 @@
   },
   'V8 Linux64 ASAN': {
     'swarming_dimensions' : {
-      'os': 'Ubuntu-16.04',
+      'os': 'Ubuntu-18.04',
     },
     'tests': [
       {'name': 'test262', 'shards': 5},
@@ -1365,7 +1347,7 @@
   },
   'V8 Linux64 TSAN': {
     'swarming_dimensions' : {
-      'os': 'Ubuntu-16.04',
+      'os': 'Ubuntu-18.04',
     },
     'tests': [
       {'name': 'benchmarks'},
@@ -1379,7 +1361,7 @@
   },
   'V8 Linux64 TSAN - stress-incremental-marking': {
     'swarming_dimensions' : {
-      'os': 'Ubuntu-16.04',
+      'os': 'Ubuntu-18.04',
     },
     'swarming_task_attrs': {
       'expiration': 14400,
@@ -1410,7 +1392,7 @@
   },
   'V8 Linux64 TSAN - isolates': {
     'swarming_dimensions' : {
-      'os': 'Ubuntu-16.04',
+      'os': 'Ubuntu-18.04',
     },
     'tests': [
       {'name': 'v8testing', 'test_args': ['--isolates'], 'shards': 7},
@@ -1418,7 +1400,7 @@
   },
   'V8 Linux64 TSAN - no-concurrent-marking': {
     'swarming_dimensions' : {
-      'os': 'Ubuntu-16.04',
+      'os': 'Ubuntu-18.04',
     },
     'tests': [
       {