chore!: Remove 1.23 deprecations

[hoxbro]

Remove the 1.23 deprecated functionality:

• Only support HOLOVIEWSRC environment variable for custom config
• Remove lazy_isinstance, not used anymore, and was only available in a few releases.
• Remove old location of IPTestCase (scheduled for removal in 1.25.0, as functionality is in IPythonCase)
• Remove streamz support.

This was deprecated in 1.21.0 (released June 2025)

Resolves #5412 (streamz issue)
Resolves #4816 (streamz issue)
Resolves #5202 (streamz issue)

[codecov]

Codecov Report

❌ Patch coverage is 18.75000% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.27%. Comparing base (776ba03) to head (b2d1529).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
holoviews/util/__init__.py	21.42%	11 Missing ⚠️
holoviews/streams.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6782      +/-   ##
==========================================
+ Coverage   89.23%   89.27%   +0.04%     
==========================================
  Files         340      340              
  Lines       73207    73164      -43     
==========================================
- Hits        65327    65318       -9     
+ Misses       7880     7846      -34     

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[philippjfr]

Great!

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.