Remove all references to the stable branch

[nodiscc]

Since v0.12.2, the stable Docker tag has been removed, but the stable branch is still present and referred to in various places. Users who want to consume the latest release should now use the release branch, which is considered stable and well tested.

All references to the stable branch should be removed and replaced with release, ideally preserving an automatic upgrade path for users who are still use it.

~/GIT/Shaarli (master=)$ git grep stable
README.md:[![](https://img.shields.io/badge/stable-v0.11.1-blue.svg)](https://github.com/shaarli/Shaarli/releases/tag/v0.11.1)
[...]
application/helper/ApplicationUtils.php:    public static $GIT_BRANCHES = ['latest', 'stable'];
application/helper/ApplicationUtils.php:        $branch = 'stable'
[...]
application/legacy/LegacyUpdater.php:            $branch = 'stable';
[...]
doc/md/Docker.md:- `stable` and `master`: **deprecated**. These tags are no longer maintained and may be removed without notice
[...]
doc/md/Shaarli-configuration.md:        "check_updates_branch": "stable",
doc/md/Shaarli-configuration.md:        "check_updates_branch": "stable",
doc/md/Shaarli-configuration.md:- **check_updates_branch**: Git branch used to check updates (e.g. `stable` or `master`).
[...]
doc/md/Upgrade-and-migration.md: * [new branch]      stable     -> origin/stable
doc/md/Upgrade-and-migration.md:### Step 2: use the stable community branch
doc/md/Upgrade-and-migration.md:$ git checkout origin/stable -b stable
doc/md/Upgrade-and-migration.md:Branch stable set up to track remote branch stable from origin.
doc/md/Upgrade-and-migration.md:Switched to a new branch 'stable'
doc/md/Upgrade-and-migration.md:* stable 890afc3 [origin/stable] Merge pull request #509 from ArthurHoaro/v0.6.5
[...]
tests/utils/config/configJson.json.php:        "check_updates_branch": "stable",

I can see the stable branch is still used as a reference in LegacyUpdater.php so we might have to keep it until the next release/update it one last time.

[ArthurHoaro]

I can see the stable branch is still used as a reference in LegacyUpdater.php so we might have to keep it until the next release/update it one last time.

Unless it triggers a real error, LegacyUpdate should not be updated. The change included in this function will be overridden by another Updater method removing that setting.