Merge branch 'ps/3.0-remote-deprecation'

Following the procedure we established to introduce breaking
changes for Git 3.0, allow an early opt-in for removing support of
$GIT_DIR/branches/ and $GIT_DIR/remotes/ directories to configure
remotes.

* ps/3.0-remote-deprecation:
  remote: announce removal of "branches/" and "remotes/"
  builtin/pack-redundant: remove subcommand with breaking changes
  ci: repurpose "linux-gcc" job for deprecations
  ci: merge linux-gcc-default into linux-gcc
  Makefile: wire up build option for deprecated features

.github/workflows/main.yml

@@ -265,9 +265,8 @@ jobs:
           - jobname: linux-reftable
             cc: clang
             pool: ubuntu-latest
-          - jobname: linux-gcc
+          - jobname: linux-breaking-changes
             cc: gcc
-            cc_package: gcc-8
             pool: ubuntu-20.04
           - jobname: linux-TEST-vars
             cc: gcc
@@ -285,9 +284,6 @@ jobs:
           - jobname: osx-meson
             cc: clang
             pool: macos-13
-          - jobname: linux-gcc-default
-            cc: gcc
-            pool: ubuntu-latest
           - jobname: linux-leaks
             cc: gcc
             pool: ubuntu-latest