Remove deprecated dt.weekofyear and dt.week accessors

[mohansree14]

Description

Removes the DataArray.dt.weekofyear and DataArray.dt.week accessors. They have raised a FutureWarning since v0.16.2 (#4534). Users should call .dt.isocalendar().week instead.

This is one of the items in the "Top 10" checklist in #11721.

• Removed both properties from DatetimeAccessor.
• Removed the unused "weekofyear" entry from _DATETIMEINDEX_COMPONENTS.
• Removed both from doc/api/deprecated.rst and doc/api-hidden.rst.
• Removed the week/weekofyear special cases from test_field_access and test_dask_field_access. isocalendar().week keeps its coverage in test_isocalendar and test_isocalendar_dask.

Checklist

• Part of Tracking: open deprecations #11721
• User visible changes (including notable bug fixes) are documented in whats-new.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 Code

[This is Claude Code on behalf of Mohansree Vijayakumar]