[test] Upgrade fyi builders to Bionic

No-Try: true
Bug: chromium:1199430
Change-Id: I2a2b04fae6c647602a1471a0c9959324e15ad37e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2846890Reviewed-by: Liviu Rau <liviurau@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74140}

infra/testing/builders.pyl

@@ -375,7 +375,7 @@
   },
   'v8_linux64_fyi_rel_ng_triggered': {
     'swarming_dimensions' : {
-      'os': 'Ubuntu-16.04',
+      'os': 'Ubuntu-18.04',
     },
     'tests': [
       # Infra staging.
@@ -1190,7 +1190,7 @@
   },
   'V8 Linux64 - debug - fyi': {
     'swarming_dimensions' : {
-      'os': 'Ubuntu-16.04',
+      'os': 'Ubuntu-18.04',
     },
     'tests': [
       # Infra staging.
@@ -1221,7 +1221,7 @@
   },
   'V8 Linux64 - fyi': {
     'swarming_dimensions' : {
-      'os': 'Ubuntu-16.04',
+      'os': 'Ubuntu-18.04',
     },
     'tests': [
       # Infra staging.