Improve readme, update xml-crypto, fix ci.

[leMaik]

I'm currently using a fork of this library, so I guess it's time to contribute my changes back to upstream. 🙃

• createCipher and createDecipher are deprecated in Node.js and cause warnings. This resolves the warnings without changing the logic. Changing this to the more secure createCipheriv and createDecipheriv would require storing IVs and break existing key files, so I kept it as is for now (it was changed to a wrong implementation in an earlier version of this PR)
• xml-crypto 4.x has a critical vulnerability, so I updated to 6.x. This in turn requires Node.js 16 or later, so I bumped the minimum version of this library in the readme (given that the oldest still supported Node.js version is Node.js 18, I hope that this is fine; if we update to Node.js >= 21, we could even get rid of rock-req and use the built-in fetch instead).
• The initialization process involved a lot of trial-and-error for me. I extended the README to include my findings, and help new users setup their EBICS accounts.

[leMaik]

Node.js 19 EOL'd over a year ago and doesn't even know the CVE that is causing problems. I'll remove it from the GitHub workflow and add Node.js 22 (upcoming LTS) instead. 18/20/22 are the currently supported versions.

Edit: I have no idea why npm ci fails for Node.js 22. It works fine locally.

[nanov]

This is incredibly good work, still trying to find out with build fails on node 22