[test] Update Mac testing from 10.13 to 10.15

Bug: chromium:1174040
Change-Id: I2ee539e5074707ac93f97a9a3a131e9c2745cdbe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2672180Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72504}

infra/testing/builders.pyl

@@ -646,7 +646,7 @@
   'v8_mac64_asan_rel_ng_triggered': {
     'swarming_dimensions' : {
       'cpu': 'x86-64',
-      'os': 'Mac-10.13',
+      'os': 'Mac-10.15',
     },
     'tests': [
       {'name': 'v8testing', 'shards': 4},
@@ -655,7 +655,7 @@
   'v8_mac64_dbg_ng_triggered': {
     'swarming_dimensions' : {
       'cpu': 'x86-64',
-      'os': 'Mac-10.13',
+      'os': 'Mac-10.15',
     },
     'tests': [
       {'name': 'mozilla'},
@@ -667,7 +667,7 @@
   'v8_mac64_gc_stress_dbg_ng_triggered': {
     'swarming_dimensions' : {
       'cpu': 'x86-64',
-      'os': 'Mac-10.13',
+      'os': 'Mac-10.15',
     },
     'tests': [
       {'name': 'd8testing', 'test_args': ['--gc-stress'], 'shards': 4},
@@ -676,7 +676,7 @@
   'v8_mac64_rel_ng_triggered': {
     'swarming_dimensions' : {
       'cpu': 'x86-64',
-      'os': 'Mac-10.13',
+      'os': 'Mac-10.15',
     },
     'tests': [
       {'name': 'mozilla'},
@@ -718,7 +718,7 @@
   'v8_mac_arm64_sim_rel_ng_triggered': {
     'swarming_dimensions' : {
       'cpu': 'x86-64',
-      'os': 'Mac-10.13',
+      'os': 'Mac-10.15',
     },
     'tests': [
       {'name': 'v8testing', 'shards': 8},
@@ -727,7 +727,7 @@
   'v8_mac_arm64_sim_dbg_ng_triggered': {
     'swarming_dimensions' : {
       'cpu': 'x86-64',
-      'os': 'Mac-10.13',
+      'os': 'Mac-10.15',
     },
     'tests': [
       {'name': 'v8testing', 'shards': 8},
@@ -736,7 +736,7 @@
   'v8_mac_arm64_sim_nodcheck_rel_ng_triggered': {
     'swarming_dimensions' : {
       'cpu': 'x86-64',
-      'os': 'Mac-10.13',
+      'os': 'Mac-10.15',
     },
     'tests': [
       {'name': 'v8testing', 'shards': 8},
@@ -1299,7 +1299,7 @@
   'V8 Mac64': {
     'swarming_dimensions': {
       'cpu': 'x86-64',
-      'os': 'Mac-10.13',
+      'os': 'Mac-10.15',
     },
     'tests': [
       {'name': 'mozilla'},
@@ -1311,7 +1311,7 @@
   'V8 Mac64 - debug': {
     'swarming_dimensions': {
       'cpu': 'x86-64',
-      'os': 'Mac-10.13',
+      'os': 'Mac-10.15',
     },
     'tests': [
       {'name': 'mozilla'},
@@ -1323,7 +1323,7 @@
   'V8 Mac64 ASAN': {
     'swarming_dimensions': {
       'cpu': 'x86-64',
-      'os': 'Mac-10.13',
+      'os': 'Mac-10.15',
     },
     'tests': [
       {'name': 'v8testing', 'shards': 5},
@@ -1332,7 +1332,7 @@
   'V8 Mac64 GC Stress': {
     'swarming_dimensions': {
       'cpu': 'x86-64',
-      'os': 'Mac-10.13',
+      'os': 'Mac-10.15',
     },
     'tests': [
       {'name': 'd8testing', 'test_args': ['--gc-stress'], 'shards': 4},
@@ -1357,7 +1357,7 @@
   'V8 Mac - arm64 - sim - debug': {
     'swarming_dimensions' : {
       'cpu': 'x86-64',
-      'os': 'Mac-10.13',
+      'os': 'Mac-10.15',
     },
     'swarming_task_attrs': {
       'expiration': 14400,
@@ -1371,7 +1371,7 @@
   'V8 Mac - arm64 - sim - release': {
     'swarming_dimensions' : {
       'cpu': 'x86-64',
-      'os': 'Mac-10.13',
+      'os': 'Mac-10.15',
     },
     'swarming_task_attrs': {
       'expiration': 14400,