feat(docs): mark cpp14 as maintenance branch

[dorotaphanSiili]

the main development branch is now cpp17 template

Closes #

📑 Description

✅ Checks

• [ ] My pull request adheres to the code style of this project
• [ ] My code requires changes to the documentation
• [ ] I have updated the documentation as required
• [ ] All the tests have passed

ℹ Additional Information

[w4bremer]

Probably should be merged once C++17 is ready and available