Merge branch '417846-slack-integration-button-migration' into 'master'

Migrate the GitLab for Slack app installation button to pajamas

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/126466



Merged-by: Justin Ho Tuan Duong <hduong@gitlab.com>
Approved-by: Dan Mizzi-Harris <dmizzi-harris@gitlab.com>
Approved-by: Lukas 'Eipi' Eipert <leipert@gitlab.com>
Approved-by: Justin Ho Tuan Duong <hduong@gitlab.com>
Reviewed-by: Lukas 'Eipi' Eipert <leipert@gitlab.com>
Reviewed-by: Sam Beckham <sbeckham@gitlab.com>
Co-authored-by: Sam Beckham <sbeckham@gitlab.com>
Co-authored-by: Lukas 'Eipi' Eipert <leipert@gitlab.com>