docs: Remove obsolete maint/release.md document

Considering that the release process is now fully describe in
"developers_guide/release_process.md" and ".gitlab/issue_templates/new-release.md",
this commit removes the obsolete document "maint/release.md".

More specifically the following section were obsolete:

* List of maintainers was outdated

* GitLab: Milestone is now automatically created in the issue template thanks
  to the /label quick action.
  See https://docs.gitlab.com/ee/user/project/quick_actions.html

* Announcements: Content of the message was last used for VTK 9.0. Since
  then wording has been updated.

* Distributions: Updated details have been added "evelopers_guide/release_process.m"
  in previous commit.

* Release Notes Emails: Scripts have been removed in
  0ef3708f (Utilities/Maintenance: remove old release maint scripts)

* Remaining steps (Creating Tarballs, Source, Documentation, Binaries,
  Uploading, Updating the Website, Announcing) are all tracked in the
  "new-release" issue template.

* Updating the Wiki: The lists of API changes were last computed between
  version 6.0 and 6.1.