crypto/tls: boringcrypto restricts RSA key sizes to 2048 and 3072 [1.19 backport]

[gopherbot]

@rsc requested issue #41147 to be considered for backport to the next 1.19 minor release.

@gopherbot please backport

[gopherbot]

Change https://go.dev/cl/449016 mentions this issue: [release-branch.go1.19] crypto/x509: allow BoringCrypto to use 4096-bit keys

[gopherbot]

Closed by merging 861ba02 to release-branch.go1.19.