[stable5.1] fix(freebusy): slot header format not respecting user's locale

[backportbot]

Backport of PR #6666

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 14.54%. Comparing base (ed77f5b) to head (fe6dfe3).
Report is 5 commits behind head on stable5.1.

Additional details and impacted files

@@            Coverage Diff             @@
##           stable5.1    #6669   +/-   ##
==========================================
  Coverage      14.54%   14.54%           
==========================================
  Files            210      210           
  Lines           9820     9820           
  Branches        2298     2310   +12     
==========================================
  Hits            1428     1428           
  Misses          8065     8065           
  Partials         327      327           

Flag	Coverage Δ
javascript	14.54% <ø> (ø)

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