KeyGenerator is used in other contexts, and we cannot change its
output... even if it does accidentally default to generating excess key
material for our primary internal usage.
ruby < 2.4 allowed accepting these values, as extra key bits were ignored. Since ce635262f5 this now has a strict checking on key length.
Default to key length 32 bytes, to match the compatible length for aes-256-cbc
Fixes#25185
This will be used to derive keys from the secret and a salt, in order to allow us to
do things like encrypted cookie stores without using the secret for multiple
purposes directly.
