Fix icrs serialization selecting incorrect tag

[braingram]

Fixes: #334

Update icrs handling converters to correctly serialize using the latest tag instead of 1.0.0. Add test to confirm serialization uses a >1.0.0 tag.

Remove test_legacy_icrs_serialize that directly used a LegacyICRSConverter.to_yaml_tree which was never expected to be used based on the comment:

asdf-astropy/asdf_astropy/converters/coordinates/frame.py

Lines 58 to 59 in 0b8f857

	# Leave the types list empty so that the 1.1.0 ICRS converter
	# is used on write.

and with this PR is now unused (and removed).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.06%. Comparing base (0b8f857) to head (3300127).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #335      +/-   ##
==========================================
- Coverage   99.06%   99.06%   -0.01%     
==========================================
  Files          71       71              
  Lines        2572     2564       -8     
==========================================
- Hits         2548     2540       -8     
  Misses         24       24              

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