Bundle version 8 checklist

[scheb]

This issue is to keep track of my todo list for the upcoming bundle version 8 (Symfony 8 support) 😀

The plan is, as usual:

• A new version of the bundle is released shortly after Symfony 8 is officially released. This typically happens in the same time frame when a new PHP version is released as well. So please be patient, until work is finished.
• A temporary 8.0-dev branch will eventually appear, with an ⚠unstable development state. It is not recommended to be used in production scenarios, as there will a lot of changes and rebasing on this branch. Use it at your own risk.

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Features

• [x] Change security factory priority to < -50 (-100?) -> #265
• [ ] Rework bundle class to use AbstractBundle

Codebase upgrades

• [ ] Remove deprecated properties/methods
• [ ] Upgrade codebase to PHP 8.x (?) style where possible
• [ ] Upgrade code examples in the docs with PHP 8.x (?) code style
• [ ] Upgrade notes
• [ ] Clean up code that is no longer needed and remove compatibility layers
• [ ] Update CONTRIBUTING.md on language features to be used
• [ ] Update .symfony.bundle.yaml
• [ ] Update version number and compatible Symfony versions in 8.x docs
• [ ] Drop old versions of dependencies
• [ ] Remove error when the old security system is used (no longer possible)
• [ ] Update mentions of "7.x" to "8.x" on the 8.x branch (i.e. docs and issue templates and gitsplit workflow)
• [ ] Update version guidance on 8.x branch
• [ ] Clean up the app
• [ ] Upgrade code style
• [ ] Check for skipped tests, whether these are still needed
• [ ] Working integration tests
• [ ] Update gitsplit config

Pre-Release Cleanup (once Symfony 8 stable is released)

• [ ] Remove minimum-stability from composer.json files
• [ ] Remove @dev from composer.json files
• [ ] Rewrite commits to clean up fiddling with CI and dependencies

Release

• [ ] Change default branch on Github to 8.x
• [ ] Release 🥳
• [ ] Abandon version 6.x, update version guidance
• [ ] Update version guidance on 7.x branch

Post-Release

• [ ] Remove .symfony.bundle.yaml from 7.x
• [ ] Change default branch of sub-packages to 8.x