credentials/tls: default GRPC_ENFORCE_ALPN_ENABLED to true

[arjan-bal]

Related issue: #434

RELEASE NOTES:

• Clients and servers will reject TLS connections that don't support ALPN. This can be disabled by setting environment variable GRPC_ENFORCE_ALPN_ENABLED to false (case insensitive). Please file a bug if you encounter any issues with this behaviour. The environment variable to revert this behaviour will be removed in an upcoming release.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.75%. Comparing base (005b092) to head (1b94310).
Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7535      +/-   ##
==========================================
- Coverage   81.91%   81.75%   -0.17%     
==========================================
  Files         361      361              
  Lines       27825    27825              
==========================================
- Hits        22794    22748      -46     
- Misses       3841     3869      +28     
- Partials     1190     1208      +18     

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

... and 19 files with indirect coverage changes

[dfawley]

Let's wait a few weeks for this since we have a lot of other semi-risky things in flight right now.

[arjan-bal]

Waiting to fix internal users before submitting this PR.

[atollena]

Small note on the release note:

Clients and servers will reject TLS connections that don't support ALPN. This can be disabled by setting environment variable GRPC_ENFORCE_ALPN_ENABLED to false (case insensitive). Please file a bug if you encounter any issues with this behaviour. This will become the default behaviour in an upcoming release.

Is the last sentence correct? Shouldn't it just read "The environment variable to revert this behavior will be removed in an upcoming release."?

[arjan-bal]

Small note on the release note:

Clients and servers will reject TLS connections that don't support ALPN. This can be disabled by setting environment variable GRPC_ENFORCE_ALPN_ENABLED to false (case insensitive). Please file a bug if you encounter any issues with this behaviour. This will become the default behaviour in an upcoming release.

Is the last sentence correct? Shouldn't it just read "The environment variable to revert this behavior will be removed in an upcoming release."?

You're correct. It was a copy/paste error. Updated the release note