Change to f-strings

[mattlorimor]

This changes all string replacements to be done via the more-contemporary f-string way.

Things to double check

• [ ] each f-string starts with an f
• [ ] nested f-strings function as expected (i think there was only one)
• [ ] the multi-line f-string doing the header build works as expected

I'll call out in the PR diff places to triple check.

For Site Improvements

• [x] Check "Allow edit from maintainers" option in pull request so that additional changes can be pushed by the Bug Alert team if needed.
• [x] I have performed a self-review of my own code.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [ ] My changes generate no new warnings.
• [ ] I have run Pelican locally to ensure my changes have not disrupted the look, feel, or functionality of the site.
• [x] I have checked my code and corrected any misspellings.
• [x] I have added a pull request description which details the nature of this change.

[mattlorimor]

Merged in main and resolved the conflicts.

I also cleaned up some explicit open and close operations and changed them to with statements. Double check those to make sure I didn't break something expecting it to be done that way.