Upgrade eslint and use flat config

[zoran995]

What this PR does

Migrate to eslint flat config as eslint removed support for legacy configs in v10.

Test me

How should reviewers test this? (Hint: If you want to provide a test catalog item, create a Gist of its catalog JSON, add its url and your branch name to this url: http://ci.terria.io/<branch name>/#clean&<raw url of your gist> , and then paste that in the body of this PR)

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.

[pjonsson]

ESLint 9 automatically warns for --report-unused-disable-directives, so that parameter could possibly be dropped from the lint function below.

[pjonsson]

I think I measured runtime when I did the ESLint 9 config, and "**/*.{jsx,tsx,js,ts,cjs,mjs}" (no spaces after the commas) is faster than a list of multiple globs.