etcd: update to 3.6.0

[tormath1]

After 4 years without a release feature, there is a new etcd release: 3.6.0

On-hold for this breaking change:

The following flags have been removed:
    --enable-v2
    --experimental-enable-v2v3

• [x] Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• [ ] Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.