Fetch server public key during OIDC auth

[fm3]

OpenID Connect authorization now fetches the server’s public key automatically. The config keys singleSignOn.openIdConnect.publicKey and singleSignOn.openIdConnect.publicKeyAlgorithm are now unused. 7267

Steps to test:

• Set up OIDC as described in Add OIDC authentication #6534 (comment) and Add client_secret to oidc requests #7263
• Should still work now even with publicKey not set in the config.
• I also tested that if a different publicKey is used, the signed token is not accepted (I added .add(BigInteger.ONE) to the parsed publicKey modulus

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• Updated changelog
• Updated migration guide if applicable

[normanrz]

Cool stuff!