Switched repo to latest ecmaVersion, except for apps still on requirejs

[orangejenny]

Technical Summary

This updates ESLint to use more modern syntax, while retaining the old ecmaVersion for areas that depend on requirejs. The requirejs parser specifically can't handle trailing commas in functions, which are flagged as an error by version 6, so it's useful to have requirejs-dependent code still on that version.

This will result in some new lint errors. I'm not going to proactively address those right now. When I made this change in the web apps webpack PR (#35594), the main change was that ESLint will now flag multi-line function calls that don't use a trailing comma after the last argument (which can be handled automatically by running ESLint with --fix).

This should resolve the kind of lint errors that come up here.

Safety Assurance

Safety story

Only affects linting.

Automated test coverage

No

QA Plan

No

Rollback instructions

• This PR can be reverted after deploy with no further considerations

Labels & Review

• Risk label is set correctly
• The set of people pinged as reviewers is appropriate for the level of risk of the change

[MartinRiese]

Hopefully that makes my IDE shut up about this too.