Make last_commit_spec w/bootstrap-vue 2

What does this MR do?

This MR lays the groundwork for the BootstrapVue upgrade that will be done in this MR: !18913 (merged)

Due to some changes in the BTooltip and BPopover components, some tests started failing after the upgrade. Those tests need to be updated accordingly:

• sync needs to be set to false
• attachToDocument must be true

This MR focuses on upgrading the tests located in spec/frontend/repository/components/last_commit_spec.js.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• [-] Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content