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 78.90%. Comparing base (0921c92) to head (4f4cf3e).
Report is 2 commits behind head on main.

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

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15225      +/-   ##
==========================================
+ Coverage   78.82%   78.90%   +0.08%     
==========================================
  Files         158      158              
  Lines        8284     8274      -10     
==========================================
- Hits         6530     6529       -1     
+ Misses       1754     1745       -9     

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[robhudson]

r+

Do you plan to split into separate branches to deploy part 1 and part 2?

[pmac]

My plan was just to land this, and then push the first commit to prod, and then the 2nd.