Upgrade eslint to 9

[sandbergja]

What maintenance needs to be done?

Upgrade eslint from 8 to 9.

Level of urgency

• [ ] High
• [x] Moderate
• [ ] Low

Why is this maintenance needed?

• So that we can use the most up-to-date versions of rules, and not get stuck on an old version

Acceptance criteria

• [x] Our eslint configuration is in a flat configuration file
• [x] We use eslint 9.0.0 or higher
• [x] Eslint still passes
• [x] If we intentionally break a lint rule, eslint still complains

Implementation notes, if any

• We need to wait until eslint-plugin-vuejs-accessibility supports eslint 9 (see https://github.com/vue-a11y/eslint-plugin-vuejs-accessibility/pull/1073)
• Eslint migration guide

[christinach]

PR in progress