Adopt SemVer for 7.0.0 and migrate release template to a gitlab issue template

Summary

• Officially adopt semver starting at OTB 7.0.0 (which may or may not be the next release)
• Document it in CONTRIBUTING.md
• Update the release guide accordingly
• Migrate the release guide from the old wiki to a gitlab issue template

Rationale

See #1603 (closed).

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

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***Check before merging:*** - All discussions are resolved - At least 2 votes from core developers, no vote. - The feature branch is (reasonably) up-to-date with the base branch - Dashboard is green - Copyright owner has signed the ORFEO ToolBox Contributor License Agreement