This updates to `@gitlab/eslint-plugin@6`, which includes breaking
changes via the transitive dependency `eslint-plugin-vue` being upgraded
from v6 to v7.

This adds/improves various lint rules of Vue files; the full changes are
documented in their [release post][1]. The most notable addition is the
`vue/no-mutating-props` rule, which can help to prevent subtle runtime
bugs.

For several of the new/changed rules, the GitLab codebase has a number
of violations. Rather than fixing all of them in this change, those new
rules are disabled (e.g., `vue/no-mutating-props`), and the changed
rules are reconfigured to mimic v6 settings (e.g.,
`vue/order-in-components`).  This way, these can be addressed/fixed
separately in follow ups.

Otherwise, one-off violations, (e.g., of `vue/require-render-return`)
have been fixed here.

There were a few odd findings during this process that might suggest
upstream bugs:

1. The `vue/one-component-per-file` rule finds what seems to be only
   false positives in this codebase, e.g.,
   `~/registry/explorer/components/list_page/cli_commands.vue`; it
   _seems_ like the `Tracking.mixin` call is being considered as another
   component definition(?).
2. Three rules were violated by the line
   `spec/frontend/clusters/components/applications_spec.js:547`:
   `@gitlab/vue-require-required-key`, `vue/require-default-prop` and
   `vue/require-prop-type-constructor.` This is odd, because no Vue
   component was being defined or constructed there. Changing a `props`
   field to `propsData` resolved all three.

Addresses #296868, part of
&5142.

[1]: https://github.com/vuejs/eslint-plugin-vue/releases/tag/v7.0.0