Update labeler GitHub workflows

[namurphy]

This repository makes use of the labeler GitHub Action, which labels pull request based on what files are being changed. This action saves us a lot of time because we don't need to label issues manually, while resulting in much more consistently applied labels. The switch to an src layout in #2598 broke some of the pull request labeler specifications. This PR fixes them, and makes some new additions.

I updated the changelog guide for better consistency with labels, and also to update the towncrier build command which was broken.

I also updated the labels to be removed after a pull request is merged.

[namurphy]

📝 This change makes it so that changes to any file in src/plasmapy/analysis or tests/analysis makes it so that this label gets applied.

[namurphy]

📝 The ** here represents any number of directories, including zero.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.23%. Comparing base (2af0dba) to head (6eeab9c).

❗ Current head 6eeab9c differs from pull request most recent head 2471b95. Consider uploading reports for the commit 2471b95 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2690   +/-   ##
=======================================
  Coverage   95.23%   95.23%           
=======================================
  Files         104      104           
  Lines        9435     9435           
  Branches     2159     2159           
=======================================
  Hits         8985     8985           
  Misses        273      273           
  Partials      177      177           

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[namurphy]

Merging this now to be able to test if it works. (One of these years, I'm going to learn how to test GitHub Actions locally...)