Upgrade to Bootstrap 5

[jcoyne]

Bootstrap 4 was EOL over a year and a half ago and is unsupported. https://getbootstrap.com/docs/4.6/end-of-life/

[corylown]

Bootstrap 4 to 5.3 migration guide: https://getbootstrap.com/docs/5.3/migration/
Example of work done to add support for Bootstrap 5 to Blacklight: https://github.com/projectblacklight/blacklight/pull/2459/files
Since we're planning a major version update for Spotlight (from 3 to 4) now may be a good time to drop support for Bootstrap 4 in Spotlight.

[corylown]

Looks like we might also need to update blacklight-gallery for Bootstrap 5 https://github.com/search?q=repo%3Aprojectblacklight%2Fblacklight-gallery%20data-slide&type=code

WIP here projectblacklight/blacklight-gallery#144

[corylown]

WIP for Spotlight: main...bootstrap-5

Still to do:

• Commented out css needs review for updated bootstrap variables or different approach
• Carousel related JS needs to work with both Bootstrap 4 or 5.
• Figure out what else I've missed causing bootstrap button/dropdowns not to work
• Style adjustments

[corylown]

Closed by #3075