[stable/kube-lego] Update link to migration guide

[rolandboon]

What this PR does / why we need it:
The docs of cert-manager are moved from the repo to readthedocs.io. The current link to migration guide in readme is dead.

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[rolandboon]

/assign @prydonius

[timstoop]

Charts are immutable, so all changes, even documentation changes, require a bump of the version in Chart.yaml. Simply increase the patch level by 1.

[prydonius]

@rolandboon the CI requires a home and apiVersion field be added to the Chart.yaml. Even though this chart is deprecated, the migration guide is an important link that we should update. We can try adding the home and apiVersion here, if there are further issues that aren't an easy fix, we may consider bypassing CI.

[unguiculus]

/ok-to-test

[unguiculus]

/assign

[unguiculus]

/lgtm

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