envconfig: default GRPC_ENFORCE_ALPN_ENABLED to false

CRDB at v1.68.0 fails to communicate with CRDB at v1.56.3 due to this
check. This is strange, since CRDB uses gRPC throughout, and in both
v1.56.3 and v1.68.0 uses `tls.NewConfig` which ensures that `NextProtos`
always contains `h2`.

gRPC introduced this check in grpc#7535.

See: cockroachdb/cockroach#136278 (comment)

Author: tbg

internal/envconfig/envconfig.go

@@ -45,7 +45,7 @@ var (
 	// option is present for backward compatibility. This option may be overridden
 	// by setting the environment variable "GRPC_ENFORCE_ALPN_ENABLED" to "true"
 	// or "false".
-	EnforceALPNEnabled = boolFromEnv("GRPC_ENFORCE_ALPN_ENABLED", true)
+	EnforceALPNEnabled = boolFromEnv("GRPC_ENFORCE_ALPN_ENABLED", false)
 	// XDSFallbackSupport is the env variable that controls whether support for
 	// xDS fallback is turned on. If this is unset or is false, only the first
 	// xDS server in the list of server configs will be used.