[py] Add explicit stacklevel=2 to remaining warnings.warn calls - #18244

Description

When warnings.warn() is invoked without an explicit stacklevel argument, Python defaults to stacklevel=1. This causes the emitted warning to point inside Selenium's internal implementation rather than pointing to the caller's callsite that invoked the method.

Across most of py/selenium/webdriver/, warnings.warn(..., stacklevel=2) is already used consistently. However, four call sites omitted stacklevel.

This PR:

• Sets stacklevel=2 on the remaining warning calls so warnings point to the caller's stack frame.
• Explicitly adds DeprecationWarning to with_tag_name().
• Adds unit tests in py/test/unit/selenium/webdriver/support/relative_locator_tests.py.
• Resolves Ruff B028 across the entire Python bindings.

Motivation and Context

Improves developer experience and debugging by correctly attributing warnings to the user's code, adhering to Ruff B028.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.