Simple broker sends notice after disconnect

Before this change the simple broker simply removed subscriptions
upon receiving a DISCONNECT message assuming it was a result of
a client STOMP WebSocket session ending.

However, if the server-side application sends a DISCONNECT to
the broker in order to terminate a session, the STOMP WebSocket
session could remain unware without any further action. This
change ensures the simple broker sends a DISCONNECT_ACK message
downstream whenever it receives a DISCONNECT.

Issue: SPR-12288
Fix SpEL handling of function reference

These changes provide more robust handling of function
reference compilation in SpEL expressions. Prior to
this change the isCompilable check was not performing
enough visibility checks on the proposed target
function, causing bytecode to be generated that
would lead to an IllegalAccessError.
The changes also bring the argument handling for
function invocation completely inline with that used
for method invocation allowing some code to be deleted.

Many new tests are also included for function
reference compilation.

Issue: SPR-12359
Resolved package tangle in spring-expression