Deprecate the Calico ipip mode in favor of bird

[twz123]

Description

K0s's Calico mode is directly passed to Calico as its backend. Calico only knows about bird, vxlan, windows-bgp and none. From the way the ipip mode is used in k0s, I figured that it's acutally the bird backend.

Deprecate the ipip mode in k0s and pass on bird to Calico. Add constants and validation.

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Documentation update

How Has This Been Tested?

• [ ] Manual test
• [ ] Auto test added

Checklist:

• [x] My code follows the style guidelines of this project
• [x] My commit messages are signed-off
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules
• [x] I have checked my code and corrected any misspellings