Fix etcd peer URL to support ipv6

[ncopa]

IPv6 URL need to have brackets [] around the ipv6 to separate it from the port. Eg https://[xx:yy:zz]:2380

Fix this by using net/url's String() formatter.

Description

Fixes #5180

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)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist:

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

[k0s-bot]

Backport failed for release-1.29, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin release-1.29
git worktree add -d .worktree/backport-5181-to-release-1.29 origin/release-1.29
cd .worktree/backport-5181-to-release-1.29
git switch --create backport-5181-to-release-1.29
git cherry-pick -x 1469e0c5142b49d2be6abd79e56eb9580f716537

[k0s-bot]

Successfully created backport PR for release-1.30:

• [Backport release-1.30] Fix etcd peer URL to support ipv6 #5190

[k0s-bot]

Successfully created backport PR for release-1.31:

• [Backport release-1.31] Fix etcd peer URL to support ipv6 #5191