Closes: #974

It does not seem like something that is normal so a warning seems in
place. However, forbidding it does not add much value since we accept
unsollicited responses (from any idp) anyway.

Closes: #975

Change documentation from recommending 2048 bit keys to using 3072 bit
keys.

Given that we're recommending people generate ten-year keys, 2048 bit
keys are probably a bit short. Almost all commercial certificate
authorities now recommend 4096 bit keys, and eduGAIN requires 3072 bit
keys for new federations.

This change aligns the SimpleSAMLphp documentation with the lower
eduGAIN requirement, since that still meets most standards bodies
recommendations for 2028 (ten years from now). cf
https://www.keylength.com/

Out of the box, installation needs running npm install too. Subversion instructions removed by indication of @thijskh

People might just copy-paste it, while there's usually no reason
to enable this by default for new installations; specially not as
a QucikStart.

See https://github.com/simplesamlphp/simplesamlphp-module-casserver

Fixes #873