Replace git ls-remote with gh api to support actions/checkout@v6

[fruppel]

actions/checkout@v6 no longer persists git credentials in the global git config the way v4 did, causing git ls-remote calls against private or token-authenticated GitHub repos to fail with authentication errors.

Replace all git ls-remote --heads calls in shopware-version.bash with equivalent gh api calls, which authenticate via the GH_TOKEN environment variable instead of git credentials. Add a github-token input (defaulting to github.token) and expose it as GH_TOKEN in the step env. Remove the now-unnecessary git config credential setup/teardown steps.