Upgrade eslint from v8 to v9

[jrentlez]

Depends on #656.

This PR upgrades eslint from version 8 to version 9. This includes migrating to the new "flat config" format.

[jrentlez]

@joluj I'm having trouble with the reuse check, it reports this error:

reuse._util - ERROR - Could not parse 'AGPL-3.0-only','
reuse.project - ERROR - 'eslint.config.mjs' holds an SPDX expression that cannot be parsed, skipping the file
# MISSING COPYRIGHT AND LICENSING INFORMATION

The following files have no copyright and licensing information:
* eslint.config.mjs

However, eslint.config.mjs has the same identifier as every other file, so I have no idea where this comes from.

[joluj]

@georg-schwarz @rhazn Any ideas regarding the licensing information?

[rhazn]

Yes, the license checker breaks if the tag is repeated in a file an not in the header. See here for example: https://github.com/jvalue/jayvee/pull/657/files#diff-9601a8f6c734c2001be34a2361f76946d19a39a709b5e8c624a2a5a0aade05f2R115 . There is a way around it I've used before in the docs autogeneration, I'll try to look it up.

[rhazn]

See here: https://github.com/jvalue/jayvee/blob/main/apps/docs-generator/src/UserDocCategoryBuilder.ts#L54

[jrentlez]

@joluj Another strange CI error (can't even reproduce this one locally):

/home/runner/work/jayvee/jayvee/libs/language-server/eslint.config.mjs
    0:0  error  Parsing error: "parserOptions.project" has been provided for @typescript-eslint/parser.
  The file was not found in any of the provided project(s): libs/language-server/eslint.config.mjs

libs/language-server/eslint.config.mjs is clearly at the expected location.

[jrentlez]

Closed in favor of #675