xds: remove temporary environment variable for least request

[atollena]

Fixes #8228.

RELEASE NOTES: Remove the GRPC_EXPERIMENTAL_ENABLE_LEAST_REQUEST environment variable that allows disabling the least request balancer with xDS. Least request was made available by default with xDS in v1.72.0.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.13%. Comparing base (718c4d8) to head (3b1137d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8248      +/-   ##
==========================================
- Coverage   82.15%   82.13%   -0.03%     
==========================================
  Files         412      412              
  Lines       40477    40474       -3     
==========================================
- Hits        33253    33242      -11     
- Misses       5864     5866       +2     
- Partials     1360     1366       +6     

Files with missing lines	Coverage Δ
internal/envconfig/envconfig.go	100.00% <ø> (ø)
...nal/xdsclient/xdslbregistry/converter/converter.go	70.00% <ø> (-0.63%)	⬇️
...ds/internal/xdsclient/xdsresource/unmarshal_cds.go	86.72% <ø> (+0.64%)	⬆️

... and 21 files with indirect coverage changes

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[atollena]

As discussed I'll keep this PR for later and work on a change that just flips the environment variable default to true.

[arjan-bal]

@dfawley is it sufficient for one minor version (1.72) to have the flag defaulted to true before removing the env var completely? If yes, we can review and merge this.

[dfawley]

In this particular case, I think it's fine. I am not really concerned that registering this LB policy by default is going to cause problems. For other things that might be more risky, because we know our users don't always upgrade in a timely fashion, I think it's better to wait at least 2-3 releases before removing the flag.

[dfawley]

@atollena can you please take a look at the merge conflicts? Thanks!

[dfawley]

@arjan-bal please do a second review & approve.