Update CONTRIBUTING.md
Summary
The CONTRIBUTING.md file contains outdated information
What is the expected correct behavior?
- Github and git.eclipse.org workflow to be deleted
- Gitlab workflow to be introduced
- Direct link to ECA (https://www.eclipse.org/legal/ECA.php) is requested by Eclipse Foundation
Relevant logs and/or screenshots
https://gitlab.eclipse.org/eclipse/titan/titan.core/-/blob/master/CONTRIBUTING.md
Titan version
8.0.0
Platform details (OS type and version)
All
/cc @aknappqwt