envconfig: enable xDS client fallback by default

[easwars]

Interop tests are passing. Setting this to true now will ensure that xDS fallback is ON by default starting v1.70.0.

RELEASE NOTES:

• xdsclient: fallback to a secondary management server (if specified in the bootstrap configuration) when the primary is down is enabled by default. Can be disabled by setting the environment variable GRPC_EXPERIMENTAL_XDS_FALLBACK to false.

[easwars]

FYI: @purnesh42H

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.04%. Comparing base (d0716f9) to head (e8c9f6b).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7949      +/-   ##
==========================================
- Coverage   82.14%   82.04%   -0.11%     
==========================================
  Files         379      379              
  Lines       38261    38261              
==========================================
- Hits        31430    31391      -39     
- Misses       5540     5561      +21     
- Partials     1291     1309      +18     

Files with missing lines	Coverage Δ
internal/envconfig/envconfig.go	100.00% <ø> (ø)

... and 20 files with indirect coverage changes