Remove remaining references of stable

[ArthurHoaro]

Fixes #1964

All remaining stable references are unrelated (except for LegacyUpdater):

.readthedocs.yml:# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
CHANGELOG.md:> The `:stable` Docker image has been removed, please use `:release` instead.
CHANGELOG.md:  - Outdated Docker documentation for stable branch
CHANGELOG.md:- Run version check tests against the 'stable' branch
CHANGELOG.md:- Tools: check the 'stable' branch for new versions (updates)
CHANGELOG.md:  Please note that Shaarli CSS are not stable and may completely change on each version
application/legacy/LegacyUpdater.php:     * Note: Due to hardcoded paths, it's not unit testable. But one line of code should be fine.
application/legacy/LegacyUpdater.php:     * otherwise we'll check updates on the `stable` branch.
application/legacy/LegacyUpdater.php:     * Note: due to hardcoded URL and lack of dependency injection, this is not unit testable.
application/legacy/LegacyUpdater.php:            $branch = 'stable';
composer.lock:    "minimum-stability": "stable",
composer.lock:    "prefer-stable": false,
doc/md/Docker.md:sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/debian $(lsb_release -cs) stable"
doc/md/dev/Development.md:- [`debian8`](https://github.com/shaarli/Shaarli/blob/master/tests/docker/debian8/Dockerfile) - [Debian 8 Jessie](https://wiki.debian.org/DebianJessie) (oldoldstable)
doc/md/dev/Development.md:- [`debian9`](https://github.com/shaarli/Shaarli/blob/master/tests/docker/debian9/Dockerfile) - [Debian 9 Stretch](https://wiki.debian.org/DebianStretch) (oldstable)
doc/md/dev/Development.md:- This branch shouldn't be used for production as it isn't necessary stable.
tests/utils/ReferenceSessionIdHashes.php: * Testing the untestable - Session ID generation
yarn.lock:    fast-json-stable-stringify "^2.0.0"
yarn.lock:    json-stable-stringify-without-jsonify "^1.0.1"
yarn.lock:fast-json-stable-stringify@^2.0.0:
yarn.lock:  resolved "https://registry.yarnpkg.com/fast-json-stable-stringify/-/fast-json-stable-stringify-2.1.0.tgz#874bf69c6f404c2b5d99c481341399fd55892633"
yarn.lock:json-stable-stringify-without-jsonify@^1.0.1:
yarn.lock:  resolved "https://registry.yarnpkg.com/json-stable-stringify-without-jsonify/-/json-stable-stringify-without-jsonify-1.0.1.tgz#9db7b59496ad3f3cfef30a75142d2d930ad72651"

[nodiscc]

Thanks!