merge_pr_46
[Azure Pipelines] limit triggers to branches actually used (web-platform-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.