🐛 [RUMF-1062] fix support for Safari 11.0

[BenoitZugmeyer]

Motivation

Fix Replay recorder support for Safari 11.0.

Changes

Replace FormData.set (see caniuse support) usages by FormData.append

Testing

Manual, staging.

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I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #1154 (bbd7b8f) into main (ff094d8) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1154   +/-   ##
=======================================
  Coverage   89.27%   89.27%           
=======================================
  Files          94       94           
  Lines        4204     4204           
  Branches      962      962           
=======================================
  Hits         3753     3753           
  Misses        451      451           

Impacted Files	Coverage Δ
packages/rum/src/transport/send.ts	100.00% <100.00%> (ø)

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