crypto/tls: add correct names for CHACHA20_POLY1305 cipher suite constants

crypto/tls: add correct names for CHACHA20_POLY1305 cipher suite constants

The cipher suites were apparently renamed late in the standardization
process, and we picked up the legacy name. We can't remove the old
constants, but add correctly named ones.

Fixes #32061

Change-Id: I65ee25c12c10934391af88b76b18565da67453fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/205068
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>