[Meta] Road to 3.0.0

Must haves

• The symfony/expression-language and symfony/intl version constraints are outdated and can conflict with other modules that need newer versions. See #3525460: Update symfony/expression-language to v7 (Compatibility with module_builder).
• Required-field validation doesn't catch empty values properly on multi-value fields. See #3471408: not_blank_constraint_rule fail on fields with multiple values.
• Confirm and document that existing 8.x-1.1 configuration keeps working after upgrading to 3.0.0. Update issue created: #3619431: Document that 8.x-1.1 configuration keeps working via field_validation_legacy after upgrading to 3.0.0
• An invalid regex pattern shows a raw PHP warning instead of a clean form error. See #3390907: Protect / help avoid validation logic problems.

Nice to haves

Some don’t have issues created yet. Feel free to create one and tag it here as well.

• Document that the "administer field validation rule set" permission allows running custom code through the Callback and Expression rule types, so it should only go to trusted roles #3621773: Document that the "administer field validation rule set" permission allows running custom code via the Callback and Expression rule types
• Add coding standards and static analysis config for contributors.
• Clarify how the Regex rule matches values. See #3391589: Regex rule doesn't work as expected.
• Move plugin definitions from annotations to PHP attributes.

Ignore list

I'm ignoring the following for now, as I don't have time to work on them, unless someone else picks them up:

• No CI pipeline exists yet, so nothing runs tests or coding standards automatically. See #3433082: Add Gitlab CI.
• No automated tests for the validation rules added in 3.x.