grpcsync : Remove OnceFunc

[ashishksrivastava]

Remove grpcsync.OnceFunc as OnceFunc is part of standard sync lib starting go1.21

Fixes: #8016
RELEASE NOTES: N/A

[linux-foundation-easycla]

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• ✅ login: ashishksrivastava / name: Ashish Srivastava (9ce5016)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.24%. Comparing base (73e4470) to head (9ce5016).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8049      +/-   ##
==========================================
+ Coverage   82.13%   82.24%   +0.11%     
==========================================
  Files         384      383       -1     
  Lines       38927    38922       -5     
==========================================
+ Hits        31971    32013      +42     
+ Misses       5618     5583      -35     
+ Partials     1338     1326      -12     

Files with missing lines	Coverage Δ
balancer_wrapper.go	87.32% <100.00%> (+1.02%)	⬆️
internal/xds/bootstrap/tlscreds/bundle.go	82.45% <100.00%> (ø)
orca/producer.go	95.53% <100.00%> (ø)
server.go	82.78% <100.00%> (-0.08%)	⬇️
xds/internal/xdsclient/authority.go	76.87% <100.00%> (ø)
xds/internal/xdsclient/clientimpl.go	79.16% <100.00%> (ø)
xds/internal/xdsclient/pool.go	80.45% <100.00%> (ø)
xds/internal/xdsclient/transport/lrs/lrs_stream.go	75.63% <100.00%> (+5.07%)	⬆️

... and 20 files with indirect coverage changes

[arjan-bal]

LGTM, adding a second reviewer.