Warn users about a future breaking change to relax_cell in VASP recipes

[Andrew-S-Rosen]

Summary of Changes

As noted in #1887, the VASP recipes are the only ones throughout quacc with relax_cell = True by default (namely: relax_job and ase_relax_job). This can lead to unexpected behavior for users that run multiple codes in quacc. To streamline things, eventually we will make relax_cell = False by default. This would be a notable breaking change, so we will not do so for some time. The warning may be perceived as annoying, but it is perhaps worthwhile.

Requirements

• Your PR should be focused on a single feature addition or bugfix.
• Your PR should have relevant, comprehensive unit tests.
• Your PR should be on a custom branch and not be named main.

Notes

• Before contributing for the first time, read the "Guidelines" section.
• If you are an external contributor, you will see a comment from @buildbot-princeton. This is solely for the maintainers.
• When your code is ready for review, ping one of the active maintainers.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.07%. Comparing base (57f859d) to head (43d70fa).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2299   +/-   ##
=======================================
  Coverage   99.07%   99.07%           
=======================================
  Files          82       82           
  Lines        3445     3450    +5     
=======================================
+ Hits         3413     3418    +5     
  Misses         32       32           

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