crypto/tls: move defaults into defaults.go

[rsc]

On #60790, we agreed to move default configuration into defaults.go so that distributions can replace that file if needed.

[gopherbot]

Change https://go.dev/cl/558375 mentions this issue: crypto/tls: move defaults into defaults.go

[gopherbot]

Change https://go.dev/cl/587296 mentions this issue: crypto/tls: move defaults into defaults.go