Changes MergeRequestApi:

- added methods "getDiffs()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/diffs

- add methods "getDiffVersions()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/versions

- add method "getDiffVersion()" and "getOptionalDiffVersion()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/versions/:version_id

- deprecate "getMergeRequestDiffs()" and "getMergeRequestDiff()"

Fixes #993

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Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>

Correct JavaDoc for "protectTag" method to show actual information of GitLab endpoint method.

Co-authored-by: Усачев Дмитрий Дмитриевич <92usachevdd@region.cbr.ru>

Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>

Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>

# Conflicts:
#	README.md
#	pom.xml

# Conflicts:
#	pom.xml

Update Mockito and JUnit

Fixes #927

Fixes #893

* Add methods to create child epics

* Add methods to manage related epics

* #954 add include_retried for jobs api

* Apply suggestions from code review

* Fix compile error

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Co-authored-by: vladimir.molodkin <vladimir.molodkin@aliexpress.ru>
Co-authored-by: Jérémie Bresson <dev@jmini.fr>
Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>

* UserApi implement createPersonalAccessToken

* adds missing scopes
* factors out common logic

* Fix integration tests to run with Gitlab "12.9.2-ce.0"

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Co-authored-by: Malte Meister <malte@labs.ws>

* Add missing attributes in EventMergeRequest

* Fix duration and tests

* Update jersey version to 2.39.1

* Update maven enforcer plugin

The GitLab API for Jobs supports - https://docs.gitlab.com/ee/api/jobs.html. Thus adding SKIPPED enumeration.

# Conflicts:
#	src/main/java/org/gitlab4j/api/GitLabApiException.java

* Add Automatic-Module-Name manifest entry: `org.gitlab4j.api`

This gets rid of warnings on Java 9+, and makes it safer to use this library as a module.

* Fix Epic Issue link association

Fixes #729

* Change return type

Use StandardCharsets.UTF_8 instead of "UTF-8" string. This removes necessity to throw a checked exception.

Apply replacement:
* "Mockito.<Object>any()" => "Mockito.any(Object[].class)"

And apply replacement:
* "javax.ws.rs.core" => "jakarta.ws.rs.core"

TestImportExportApi: change the file length expectation