Using win 1909 for win-pixel-tester-rel

In preparation for the win 1909 update. We should get new gold images
caused by system update if there's any, and approve them.

Change-Id: Id18d2d46d4280f1096e9659b1a5b421bbe5401fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453924Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Sven Zheng <svenzheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815321}

testing/buildbot/chromium.fyi.json

@@ -53898,7 +53898,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Windows-10-15063"
+              "os": "Windows-10-18363"
             }
           ],
           "service_account": "chrome-gold@chops-service-accounts.iam.gserviceaccount.com"

testing/buildbot/waterfalls.pyl

@@ -2732,7 +2732,7 @@
         'swarming': {
           'dimension_sets': [
             {
-              'os': 'Windows-10-15063',
+              'os': 'Windows-10-18363',
             },
           ],
         },