[Azure Pipelines] limit triggers to branches actually used (web-platf…

…orm-tests#20441)

Branches like epochs/six_hourly wouldn't do anything, but they create
a lot of entries in the Azure DevOps dashboard:
https://dev.azure.com/web-platform-tests/wpt/_build?definitionId=1

The no-op builds are mostly harmless, but make it harder to find the
interesting builds.

.azure-pipelines.yml

@@ -16,8 +16,13 @@
 #    https://docs.microsoft.com/en-us/azure/devops/pipelines/agents/v2-windows
 
 trigger:
-- epochs/*
-- triggers/*
+# These are all the branches referenced in the jobs that follow.
+- epochs/daily
+- epochs/three_hourly
+- triggers/edge_dev
+- triggers/edge_canary
+- triggers/safari_stable
+- triggers/safari_preview
 
 jobs:
 # The affected tests jobs are unconditional for speed, as most PRs have one or