pickfirst: Remove old pickfirst

[arjan-bal]

Fixes: #8561
Addresses: #6472

The new pickfirst has been the default since gRPC Go v1.71.0 and all reported bugs have been fixed. This PR removes the old pickfirst policy completely.

The exported symbols in the pickfirstleaf package are retained with a deprecation notice for removal after one release.

RELEASE NOTES:

• pickfirst: Remove the old pick_first LB policy. The new pick_first has been the default since v 1.71.0.

[codecov]

Codecov Report

❌ Patch coverage is 91.06145% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.15%. Comparing base (0b57abb) to head (0b30392).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
balancer/pickfirst/pickfirst.go	91.11%	20 Missing and 11 partials ⚠️
balancer/pickfirst/pickfirstleaf/pickfirstleaf.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8672      +/-   ##
==========================================
+ Coverage   83.03%   83.15%   +0.11%     
==========================================
  Files         417      417              
  Lines       32310    32182     -128     
==========================================
- Hits        26829    26761      -68     
+ Misses       4087     4038      -49     
+ Partials     1394     1383      -11     

Files with missing lines	Coverage Δ
balancer/leastrequest/leastrequest.go	88.65% <100.00%> (ø)
balancer/ringhash/ringhash.go	94.11% <100.00%> (ø)
balancer/roundrobin/roundrobin.go	100.00% <100.00%> (ø)
balancer/weightedroundrobin/balancer.go	86.31% <100.00%> (+0.76%)	⬆️
internal/envconfig/envconfig.go	100.00% <ø> (ø)
xds/googledirectpath/googlec2p.go	88.88% <ø> (+1.83%)	⬆️
balancer/pickfirst/pickfirstleaf/pickfirstleaf.go	0.00% <0.00%> (-89.68%)	⬇️
balancer/pickfirst/pickfirst.go	89.62% <91.11%> (+50.91%)	⬆️

... and 19 files with indirect coverage changes

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[dfawley]

Deprecate the whole package?

[arjan-bal]

Done. Since this PR is removing a test workflow, it will higher extra privileges to merge.

[easwars]

I'm skipping reviewing this as it has already been approved by @dfawley.

Also, moving it back to @dfawley for the merge as it requires higher privileges to merge because of the removal of a test workflow.