balancer: enable GRPC_GO_EXPERIMENTAL_CASE_SENSITIVE_BALANCER_REGISTRIES by default

[Pranjali-2501]

Part of #5288

Follow up of PR: #8837

This PR enabled the env variable GRPC_GO_EXPERIMENTAL_CASE_SENSITIVE_BALANCER_REGISTRIES by default to transition balancer registries to be case-sensitive.

RELEASE NOTES:

• balancer: balancer registries are now case-sensitive by default. Disable GRPC_GO_EXPERIMENTAL_CASE_SENSITIVE_BALANCER_REGISTRIES env var to temporarily revert this behavior.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.82%. Comparing base (ccc8d7b) to head (169aacb).
⚠️ Report is 34 commits behind head on master.

Files with missing lines	Patch %	Lines
balancer/balancer.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9017      +/-   ##
==========================================
- Coverage   82.99%   82.82%   -0.18%     
==========================================
  Files         411      413       +2     
  Lines       32917    33404     +487     
==========================================
+ Hits        27319    27666     +347     
- Misses       4196     4253      +57     
- Partials     1402     1485      +83     

Files with missing lines	Coverage Δ
internal/envconfig/envconfig.go	100.00% <ø> (ø)
balancer/balancer.go	52.38% <0.00%> (-42.86%)	⬇️

... and 53 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[easwars]

The release notes need to be updated. They are wrong.