.github: support all stable branches

If this eventually poses a problem for unsupported branches we can fix
them directly.

Sponsored by:	Innovate UK

Reviewed by:	emaste
Differential Revision:	https://reviews.freebsd.org/D53838

(cherry picked from commit 1e18927957)

.github/workflows/cross-bootstrap-tools.yml

@@ -2,7 +2,7 @@ name: Cross-build Kernel
 
 on:
   push:
-    branches: [ main, 'stable/14', 'stable/13' ]
+    branches: [ main, 'stable/[1-9][0-9]' ]
   pull_request:
     branches: [ main ]
   workflow_dispatch: