crypto/tls: use standard chacha20-poly1305 cipher suite names

crypto/tls: use standard chacha20-poly1305 cipher suite names

The different chacha20-poly1305 cipher suites were renamed to
include the _SHA256 suffix, which is the canonical naming convention.

The occurrences of the old names were still not updated, which can lead
to confusion when searching for the canonical names in the codebase.

Change-Id: I4f90e9cbedc3552c3481c8b0c616b6f915ddd345
Reviewed-on: https://go-review.googlesource.com/c/go/+/689135
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>