Update major Node and Eslint versions

[jorg-vr]

This pull request contains an update of eslint and a required update of node as well.

It replaces #5474 whcih gets frequently closed by dependabot. See dependabot/dependabot-core#7647

As this is a major version change, this also required some changes on our side:

• I updated all github runners to node 20 as older versions are no longer supported
• I moved our .eslintrc and .eslintignore to modern eslint.config.js using this migration guide (eslintrc is no deprecated)
• I simplified the config by always using the typescript eslint config (Almost everything is typescript now anyway and the typescript specific rules dont seem to break on javascript files)
• I fixed some minor issues the linter started complaining about
• yarn lint now also explicitly lints the test directory

Before release we should

• update our production servers to node 20 (tests should run in the same context as production)
• set "no-unused-vars" rule back to error once this issue is fixed: markVariableAsUsed for the customElement decorator 43081j/eslint-plugin-lit#195
• Discuss if we want to keep a line length warning (We now have hundreds of violations, which wouldn't be improved by shorthening them in my opninon)
• Discuss if we want to enforce the 'no-explicit-any' rule. Because everything is typescript now, there is less need to use any.
• Fix my editor to use the new config

[niknetniko]

To give 2cents:

• Discuss if we want to keep a line length warning (We now have hundreds of violations, which wouldn't be improved by shorthening them in my opninon)

I am strongly in favour of dropping the warning (I already do this locally).

• Discuss if we want to enforce the 'no-explicit-any' rule. Because everything is typescript now, there is less need to use any.

I think this would be good, but it might be a lot of work.