Use flexbox for laying out the buttons, to avoid whitespace in the
template from affecting visual spacing.

This also replaces the deprecated slot syntax with modern shorthand.

Finally, this removes a snapshot test that wasn't providing any value,
since other tests in the suite make the necessary assertions about the
`Manage licenses` and `View full report` buttons in the `ReportSection`
anyway (which is what was in the snapshot).

Addresses https://gitlab.com/gitlab-org/gitlab/-/issues/283889.