protected_branches_selector does not support multi select.

Why are we doing this work

protected branch selector is useful as it can query protected branches for a project based on a search term out of the box.

However it doesn't support multi select so features that support multiple protected branches are either not utilizing its full potential or limited to the usage of a single branch.

As of now, it is currently being used as part of the form (status check related) and approval rules (under project settings). Those will also be required to be changed if we decide to go further with this change.

Scan result policy rule mode would also benefit from having the multi select support.

Multi select support would like the following:

Relevant links

Implementation plan

• frontend convert protected_branches_selector.vue to GlCollapsibleListbox
• frontend add a multiple prop that toggles on/off the GlCollapsibleListbox multiple prop

Verification

1. Verify protected branches selector works in policy editor (staging example
2. Verify protected branches selector works in all places it is used (see this previous MR for how to verify)