tests: remove obsolete < 3.2 skipif markers

why: tmux < 3.2 is now deprecated (TMUX_SOFT_MIN_VERSION = "3.2a").
All version skipif markers for < 3.0, < 3.2, <= 3.1, and < 2.9 are
now obsolete since all supported tmux versions have these features.

what:
- Remove skipif(has_lt_version("3.0")) from environment tests
- Remove skipif(has_lt_version("3.2")) from window direction tests
- Remove skipif(has_lt_version("2.9")) from resize tests
- Remove skipif(has_lte_version("3.1")) from zoom tests
- Clean up unused imports (has_lt_version, has_lte_version)

Author: tony

tests/legacy_api/test_window.py

@@ -10,7 +10,7 @@
 import pytest
 
 from libtmux import exc
-from libtmux.common import has_gte_version, has_lt_version, has_version
+from libtmux.common import has_gte_version, has_version
 from libtmux.constants import OptionScope
 from libtmux.pane import Pane
 from libtmux.server import Server