Fix petablint v2 warning

[dweindl]

Don't show Support for PEtab2.0 and all of petab.v2 is experimental warning when validating PEtab v1 problems.

Import petab.v2.* only when needed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 30.76923% with 9 lines in your changes missing coverage. Please review.

Project coverage is 74.56%. Comparing base (6fffafb) to head (1f7d993).
Report is 41 commits behind head on develop.

Files with missing lines	Patch %	Lines
petab/versions.py	36.36%	6 Missing and 1 partial ⚠️
petab/petablint.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #342      +/-   ##
===========================================
- Coverage    74.59%   74.56%   -0.03%     
===========================================
  Files           56       56              
  Lines         5518     5521       +3     
  Branches       958      959       +1     
===========================================
+ Hits          4116     4117       +1     
- Misses        1036     1039       +3     
+ Partials       366      365       -1     

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