feat: debounce rfc index refresh

[jennifer-richards]

Adds the now ubiquitous DirtyBits model for the RFC index. The API call purple used to trigger the task now just marks the index as dirty.

Renames the create_rfc_index_task to refresh_rfc_index_task. This should be schedule to run periodically (5-10 minutes, depending on how long index generation takes in practice).

(n.b., this does not yet import errata.json updates, that will come separately)

@kesara, this touches the task you're working on in #10706 but that should only be superficial and should be easy to merge.

@rjsparks, note that the third commit (931e9bc) removes some ca 2011 code that is unused as far as I can tell. I searched for imports fairly thoroughly, but if you think there's a chance it's still live I can revert that.

[rjsparks]

@rjsparks, note that the third commit (931e9bc) removes some ca 2011 code that is unused as far as I can tell. I searched for imports fairly thoroughly, but if you think there's a chance it's still live I can revert that.

I agree that this is unreachable code and it is appropriate to remove it.

Addressed the comment

[codecov]

Codecov Report

❌ Patch coverage is 46.87500% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.40%. Comparing base (68104b5) to head (b9ac30a).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
ietf/sync/tasks.py	3.44%	28 Missing ⚠️
ietf/sync/rfcindex.py	70.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10708      +/-   ##
==========================================
- Coverage   88.44%   88.40%   -0.04%     
==========================================
  Files         330      330              
  Lines       44365    44433      +68     
==========================================
+ Hits        39237    39281      +44     
- Misses       5128     5152      +24     

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