manifestlint: move from kubeval to kubeconform

[tuminoid]

kubeval is not maintained anymore, and suggest moving to kubeconform. Change manifestlint.sh to use kubeconform.

Pin kubeconform image with a version and a digest.

[metal3-io-bot]

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[tuminoid]

Note: there is chicken/egg issue with project-infra change: metal3-io/project-infra#462

[metal3-io-bot]

@tuminoid: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
manifestlint	ee2f07a	link	true	/test manifestlint

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[tuminoid]

/hold

Holding to sync merging, due chicken/egg issue.