Update dates and how optional deps are specified and tested

[rly]

Motivation

• Updated copyright dates
• Created optional dependency groups in pyproject.toml and update GitHub Actions workflows to use those instead of requirements files.
• Stopped using pinned dependencies in the docs and testing. These are not necessary for library testing, confuse new users and developers, and add maintenance burden. Current dependencies are stable enough that they need not be pinned and users can report the libraries they use.
• Stopped redundant testing using a conda environment.
• Adopted changelog format conventions: https://keepachangelog.com/en/1.1.0/

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[rly]

linkml does not yet support Python 3.13 so we will need to use Python 3.12 to test optional dependencies.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.87%. Comparing base (a7a5507) to head (977ddc2).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1230      +/-   ##
==========================================
+ Coverage   90.86%   90.87%   +0.01%     
==========================================
  Files          42       42              
  Lines        9524     9524              
  Branches     2725     1921     -804     
==========================================
+ Hits         8654     8655       +1     
  Misses        576      576              
+ Partials      294      293       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.