Use Choice config validation constraint

Hi I just tried to create a basic choice contraint following these docs https://www.drupal.org/node/3376195
https://symfony.com/doc/current/reference/constraints/Choice.html#basic-usage

Thank you for the effort, but that MR is not really helpful.

- The validation constraint definition needs to be conditionally backported, as the CKEditor5 module previously did: https://git.drupalcode.org/project/drupal/-/merge_requests/4352/diffs#18...
- The csp_directive_source_list.base schema can be a static list of options.
- csp_directive_source_list.flags could be limited to a static list of options. I don't know that the callback is given enough context to limit flags that aren't valid for each directive (e.g. unsafe-inline doesn't make sense if not on default-src, script-src, or style-src)
- csp_policy.reporting.plugin could possibly be limited by a callback to the available reporting plugins

gapple → closed merge request !15

I think easier to require ^10.2 for 2.0.0, and skip backporting support for <=10.1 in the 8.x-1.x branch.

I think having this as part of 2.1.0, after core support for 8.1 is dropped, since this isn't necessary to introduce in 2.0.0.