JUnit 5 obsolete assertions in ImmutableIntBagTest(lost-and-found-kata-solutions): Assert replaced with 'org.juit.jupiter.api.Assertions'

[prathasirisha]

Update occurrencesOf() method in ImmutableIntBagTest in lost-and-found-kata-solutions to replace 'assertEquals()' with a call to a method from Junit 5 assertions class.

Please note that this issue is created for GHC 2024 Open Source Day; thus, priority will be given to attendees of this event. After the event, this issue will be open to everyone.