doc/go1.16: add -insecure deprecation to release notes
doc/go1.16: add -insecure deprecation to release notes

Updates #37519.

Change-Id: Iddf88a24334d4740f9c40caa2354127298692eeb
GitHub-Last-Rev: deda4c858b5c5582fa63ae7f1eee5f57292670c4
GitHub-Pull-Request: golang/go#41545
Reviewed-on: https://go-review.googlesource.com/c/go/+/256419
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Trust: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Trust: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,golang/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,golang/go,nawawi/go