ci(compile_ubuntu): only run on push to main

compile_ubuntu.yml is a PR-focused Ubuntu container smoke check. On
stable, beta, and release/** branches it duplicates build_all_targets.yml,
which already covers those branches with the full board matrix and the
S3 uploads. Drop stable, beta, and release/** from the push branch list
so it matches the trigger pattern used by compile_macos.yml, checks.yml,
and clang-tidy.yml.

Fixes #27171

Signed-off-by: Ramon Roche <mrpollo@gmail.com>

.github/workflows/compile_ubuntu.yml

@@ -4,9 +4,6 @@ on:
   push:
     branches:
       - 'main'
-      - 'stable'
-      - 'beta'
-      - 'release/**'
     paths-ignore:
       - 'docs/**'
   pull_request: