chore: Replace unmaintained bootstrap-datepicker with vanillajs-datepicker

[larseggert]

Fixes #3569.

[codecov]

Codecov Report

Merging #5259 (524827d) into main (0ec1264) will increase coverage by 0.01%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##             main    #5259      +/-   ##
==========================================
+ Coverage   88.58%   88.59%   +0.01%     
==========================================
  Files         295      294       -1     
  Lines       40114    40061      -53     
==========================================
- Hits        35534    35492      -42     
+ Misses       4580     4569      -11     

Impacted Files	Coverage Δ
ietf/iesg/urls.py	100.00% <ø> (ø)
ietf/meeting/models.py	86.80% <ø> (+0.72%)	⬆️
ietf/meeting/urls.py	81.25% <ø> (ø)
ietf/iesg/views.py	92.53% <85.18%> (-0.71%)	⬇️
ietf/secr/sreq/views.py	91.37% <85.71%> (+0.21%)	⬆️
ietf/doc/mails.py	96.20% <100.00%> (ø)
ietf/doc/utils_search.py	80.58% <100.00%> (ø)
ietf/group/views.py	90.95% <100.00%> (+0.09%)	⬆️
ietf/ietfauth/views.py	92.12% <100.00%> (+1.09%)	⬆️
ietf/meeting/helpers.py	89.84% <100.00%> (-0.07%)	⬇️
... and 7 more

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