[WIP][AdminBundle] Migrate CKEditor4 to CKEditor5

[dbeerten]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	yes
Deprecations?	no
Fixed tickets	#2135

TODO:

• Allow custom config (or extended config) on project level
  Don't know if this is possible since you can't/shouldn't load different builds on the same page.
  https://ckeditor.com/docs/ckeditor5/latest/builds/guides/integration/advanced-setup.html#scenario-3-using-two-different-editors
  You can change the toolbar property for different instances, but you can't add plugins to an existing build.
• Update cookbook doc for defining custom ckeditor configuration
• Remove CKEditor4 code, ckEditorConfigs & code examples
• Internal links: Browser? (custom plugin, not default in CKEditor5)
• Reduce toolbar to essentials: no tables, no images, no headings
• Word count / Max length?
• Language UI / Content

[ProfessorKuma]

Hi @, your PR needs some changes

• It seems that you should have submitted to the next major branch.
• This PR seems to need a milestone of a major release.