Revert "protoc-gen-go-grpc: remove use_generic_streams_experimental flag (defaults to true) (#7654)

[purnesh42H]

This PR reverts #7654 which attempted to:

• Remove the use_generic_streams_experimental flag.
• Make the use of generics mandatory for stream methods.

This revert is necessary because the previous changes caused compatibility issues with existing usages of client and server stream methods within internal google3.

We will revisit this change once the affected usages in google3 have been migrated.

RELEASE NOTES: None

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.86%. Comparing base (e8a70c6) to head (24f11cd).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7703      +/-   ##
==========================================
+ Coverage   81.84%   81.86%   +0.01%     
==========================================
  Files         361      361              
  Lines       27822    27822              
==========================================
+ Hits        22772    22776       +4     
+ Misses       3854     3851       -3     
+ Partials     1196     1195       -1     

see 15 files with indirect coverage changes