Migrate ESLint config to new config format

[mikeybinns]

ESLint have a new config format which is worth migrating to. I see it taking the following steps (in no particular order):

• [ ] Update ESLint dependency to latest (^9.0.0)
• [ ] Deprecate @atomicsmash/eslint-config package the new config format no longer requires a strict package name, so we can export the new config from the @atomicsmash/coding-standards package directly.
• [ ] Create new flat config in @atomicsmash/coding-standards which matches the current config, being sure to include a peer dependency for ESLint and any extended configs we use.
• [ ] Remove changeset rule which keeps the @atomicsmash/eslint-config package at the same version as coding standards.
• [ ] Remove @atomicsmash/eslint-config as dependency of coding standards
• [ ] Write migration guide for our package that indicates how to keep using the old config if required, and also encourages users to update their local configs to flat config and the new standard.

[mikeybinns]

@oliver-stott Tagging so you're kept in the loop