refactor: remove dead zarr<3 (has_zarr_v3) guards in tests

[mvanhorn]

Summary

Removes the now-dead zarr v2 test guards built on has_zarr_v3, which can no longer be false now that xarray requires zarr>=3.0.0.

Why

xarray's lower bound for zarr is now 3.0.0, so the has_zarr_v3 flag is always true whenever zarr is installed. The skipif(not has_zarr_v3, ...) decorators and if has_zarr_v3: branches built on it are dead code that can never take the false path. Issue #11346 asks to clear out these stale 2.x guards so the test suite stops carrying conditionals that no longer mean anything.

Description

Now that xarray's lower bound for zarr is 3.0.0, the has_zarr_v3 flag in xarray/tests/__init__.py is always true whenever zarr is installed, so the skipif(not has_zarr_v3, ...) decorators and if has_zarr_v3: branches built on it never take the false path. This removes those stale 2.x guards in test_backends.py and test_backends_datatree.py and collapses the conditional in xarray/tests/__init__.py, keeping the formerly-guarded bodies. The still-meaningful version gates has_zarr_v3_dtypes (3.1.0) and has_zarr_v3_async_oindex (3.1.2) are left untouched, per issue #11346.

Checklist

• Closes dead code that conditions on zarr<3 #11346
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: Claude

[welcome]

Congratulations on completing your first pull request! Welcome to Xarray! We are proud of you, and hope to see you again!

[mvanhorn]

Thanks for the quick merge @dcherian! Nice to have the dead has_zarr_v3 guards out of the test suite.

[mvanhorn]

Much appreciated, @dcherian. Good to see the dead zarr<3 guards gone from the tests.