crypto/rsa: revert minimum GenerateKey size to 32 bits

crypto/rsa: revert minimum GenerateKey size to 32 bits

No point in causing breakage even with GODEBUG=rsa1024min=0.

Change-Id: I923254a8c8afaca77be551b19e3555c44ebdbb67
Reviewed-on: https://go-review.googlesource.com/c/go/+/636557
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>