Remove use of etags and lastmod data from sitemaps

[pmac]

These commits will need to be deployed one at a time. The 2nd one contains the migration to remove the data from the DB, while the first removes the use of the lastmod field.

• Ref Remove lastmod timestamps from the sitemap #15224: Remove use of etags and lastmod data from sitemap generation
• Ref Remove lastmod timestamps from the sitemap #15224: Remove the rest of lastmod data use and remove lastmod field from DB

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.96%. Comparing base (bb83e23) to head (1575c72).

Files with missing lines	Patch %	Lines
bedrock/sitemaps/models.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15225      +/-   ##
==========================================
+ Coverage   77.88%   77.96%   +0.08%     
==========================================
  Files         163      163              
  Lines        8480     8470      -10     
==========================================
- Hits         6605     6604       -1     
+ Misses       1875     1866       -9     

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