bump gwcs version check in test

[braingram]

gwcs 0.21.0 was released.

I believe this may be interfering with a version check in the test_read_wcs_with_backwards_affine test.

The check was previously updated in:
#313
and added in:
#305
which mentions the draft gwcs PR:
spacetelescope/gwcs#457
which might be what this test is attempting to check against. @Cadair can you confirm?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.68%. Comparing base (38b6829) to head (f662fe8).

❗ Current head f662fe8 differs from pull request most recent head abbe041. Consider uploading reports for the commit abbe041 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #346      +/-   ##
==========================================
- Coverage   97.73%   97.68%   -0.06%     
==========================================
  Files          34       34              
  Lines        1990     1990              
==========================================
- Hits         1945     1944       -1     
- Misses         45       46       +1     

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[Cadair]

Thanks @braingram 🙏