Changes

Remove hardcoded string constant representing "UTF-8" where possible

Review/cleanup Facelets/Request/Response encoding implementation
https://github.com/eclipse-ee4j/mojarra/issues/5383

Further clarified comments after self-review.

UIInput#getSubmittedValue() must also consider isLocalValueSet() when
INTERPRET_EMPTY_STRING_SUBMITTED_VALUES_AS_NULL=true
https://github.com/eclipse-ee4j/mojarra/issues/5386

Further cleanup of comments

Fixed wrong base url to validate contracts resource; it should be
validated against the path of the facelet itself not against the
resolved url of the facelet
https://github.com/eclipse-ee4j/mojarra/issues/5389

Add session mutex
https://github.com/eclipse-ee4j/mojarra/issues/5380

Fallback in case there's no mutex attribute for whatever reason

Consider ClosedChannelException as well when checking for connection
abort
https://github.com/eclipse-ee4j/mojarra/issues/5391

Mmkay instanceof var was only available since Java 14

PartialViewContextImpl should use same VisitContext to reset values as
the one used to process the execute and render
https://github.com/eclipse-ee4j/mojarra/issues/5396

Add clarifying comment wrt copy of resetValues()

resetValues should not carry EXECUTE_LIFECYCLE visit hint
https://github.com/eclipse-ee4j/mojarra/issues/5396

Revert "UIInput#getSubmittedValue() must also consider isLocalValueSet() when INTERPRET_EMPTY_STRING_SUBMITTED_VALUES_AS_NULL=true https://github.com/eclipse-ee4j/mojarra/issues/5386"

This reverts commit 081e6ea797eacb6793bfe4e813e7e7dbda96d565.

Found and fixed two more occurrences on synchronized(session)
https://github.com/eclipse-ee4j/mojarra/issues/5380

Improved impl for https://github.com/eclipse-ee4j/mojarra/issues/5375

First step, migrate junit4 to junit5

Analyzed and fixed failing UIComponentBaseBehaviorTestCase; it was
previously never invoked, so is actually activated for the first time;
it however initially failed on 2 tests:
1) testAddBehavior(): the original assumption was that adding handler on
unsupported behavior event would throw ISE but the impl never did this,
and the javadoc also didn't say any word about this, so I adjusted the
test itself
2) testGetBehaviors(): the original assumption was that
getClientBehaviors() returns a mutable collection but this was not the
case when no initial behaviors were added, and the javadoc also didn't
say any word about this, so I adjusted the test itself

migrated EasyMock/PoweMock to Mockito

Micro optimization to clarify intent of code
https://github.com/eclipse-ee4j/mojarra/issues/5408

Reduce logging level of exception which is already rethrown during
processing a lifecycle phase -- this only causes duplicate logs
https://github.com/eclipse-ee4j/mojarra/issues/5404