credentials/tls: Remove environment variable for disabling ALPN (#8660)

credentials/tls: Remove environment variable for disabling ALPN (#8660)

Related issue: https://github.com/grpc/grpc-go/issues/434

RELEASE NOTES:
* credentials/tls: Remove the `GRPC_ENFORCE_ALPN_ENABLED` environment
variable. ALPN is now enforced by default. Users who must disable ALPN
enforcement can temporarily use the [experimental transport
credentials](https://pkg.go.dev/google.golang.org/grpc@v1.76.0/experimental/credentials).
These experimental credentials will be removed in an upcoming release;
users who depend on them must vendor this version of gRPC or copy the
relevant code into their own codebase.