ref(assignee-selector): Get rid of old component for assignee selector in issue details

[MichaelSun48]

This PR swaps the old assignee selector dropdown in the issue details page for the new one that was recently introduced in the issue stream. The design of the trigger has not been changed with the exception of the chevron, which has been changed to use <Chevron> instead of <IconChevron>.

Closes #69829

Old:

New:

TODOs:

• [x] Fix old tests

[codecov[bot]]

Bundle Report

Changes will increase total bundle size by 17.1kB :arrow_up:

Bundle name	Size	Change
app-webpack-bundle-array-push	26.79MB	17.1kB :arrow_up:

[malwilley]

Looks like one of the acceptance tests is failing - you'll probably need to change how the test is selecting the assignee dropdown

[github-actions[bot]]

🚨 Warning: This pull request contains Frontend and Backend changes!

It's discouraged to make changes to Sentry's Frontend and Backend in a single pull request. The Frontend and Backend are not atomically deployed. If the changes are interdependent of each other, they must be separated into two pull requests and be made forward or backwards compatible, such that the Backend or Frontend can be safely deployed independently.

Have questions? Please ask in the #discuss-dev-infra channel.