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.
***Check before merging:*** - All discussions are resolved - At least 2