fix: Preserve Document.time when migrating rfcs

[jennifer-richards]

No description provided.

[codecov]

Codecov Report

Merging #5954 (17f53a3) into feat/rfc (518aedb) will decrease coverage by 0.07%.
The diff coverage is 77.98%.

@@             Coverage Diff              @@
##           feat/rfc    #5954      +/-   ##
============================================
- Coverage     88.65%   88.59%   -0.07%     
============================================
  Files           288      288              
  Lines         39950    40049      +99     
============================================
+ Hits          35419    35480      +61     
- Misses         4531     4569      +38     

Impacted Files	Coverage Δ
ietf/doc/views_downref.py	100.00% <ø> (ø)
ietf/group/milestones.py	86.36% <0.00%> (ø)
ietf/name/models.py	100.00% <ø> (ø)
ietf/review/utils.py	89.57% <ø> (ø)
ietf/secr/sreq/views.py	91.20% <ø> (ø)
ietf/secr/utils/group.py	91.30% <ø> (+22.88%)	⬆️
ietf/submit/forms.py	78.99% <ø> (ø)
ietf/nomcom/views.py	93.11% <3.03%> (-3.68%)	⬇️
ietf/doc/utils_search.py	80.58% <50.00%> (ø)
ietf/group/views.py	90.52% <50.00%> (-0.01%)	⬇️
... and 18 more

... and 3 files with indirect coverage changes