build: branch manager should use "7.3.x" as patch branch (#15097)

Updates the branch manager to validate patch PRs against the
`7.3.x` branch. This is necessary because currently some PRs
are marked as failing because they no longer apply to the
outdated `7.2.x` branch.

.github/branch-manager.yml

@@ -1,7 +1,7 @@
 enabled: true
 
 branches:
-  - branchName: 7.2.x
+  - branchName: 7.3.x
     label: "target: patch"
   - branchName: 7.x
     label: "target: minor"