transport/grpchttp2: revert #7477 usage of mem package

[printchard]

Revert #7477 in order to pursue a implementation with standard types

RELEASE NOTES: none

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.

Project coverage is 81.37%. Comparing base (e6b6318) to head (7b02e5f).

Files	Patch %	Lines
internal/transport/grpchttp2/framer.go	0.00%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7485      +/-   ##
==========================================
- Coverage   81.59%   81.37%   -0.23%     
==========================================
  Files         357      357              
  Lines       27224    27242      +18     
==========================================
- Hits        22213    22167      -46     
- Misses       3807     3851      +44     
- Partials     1204     1224      +20     

Files	Coverage Δ
internal/transport/grpchttp2/framer.go	0.00% <0.00%> (ø)

... and 22 files with indirect coverage changes