Remove UnmodifiableCollectionUtil and use pitest funmodifiablecollection feature

[romani]

UnmodifiableCollectionUtil as created at https://github.com/checkstyle/checkstyle/pull/13127/files
to bypass pitest issue mutation of unmodified collections.

Pitest released a fix at hcoles/pitest#1300 and we upgraded to the version of pitest at 68f7af1

We need to activate pitest feature and revert UnmodifiableCollectionUtil.

[nrmancuso]

We still have work to do here: #14484 (comment). This issue cannot be closed until we have entirely removed UnmodifiableCollectionUtil

But we can not proceed until pitest do and release more fixes. Read hcoles/pitest#1300 (comment) for more details