Cleanup: set rsa.SignPKCS1v15 RandReader input param to nil

[pgporada]

As of go 1.20, rsa.SignPKCS1v15 input parameters state that:

the random parameter is legacy and ignored, and it can be nil.

We want projects that are building go-jose with versions older than 1.20.x to continue being able to build go-jose for the foreseeable future.. After sufficient time has passed, we'll clean up this code. What does sufficient time mean? I don't know yet, but I'll probably see you in a few years. 👋🏼

[maraino]

Currently, go-jose works on the latest version, Go 1.20.2, and I don't think Go will change that in any Go 1.x version, it will break a bunch of things if they do.