Document and test upgrades from v1.4.3 and v1.5.3 to v1.6.0

[nrb]

Describe the problem/challenge you have

Full upgrade steps are not documented for v1.5.3 and v1.4.3 to v1.6.0. These should be done before final release.

Describe the solution you'd like

Documentation for upgrading to v1.6.0 which should include (but not limited to):

• Any CRD upgrades necessary
• Impacts on restic data

Anything else you would like to add:

v1.4.3 instructions should be included since requiring uses to move through 2 versions to get a working update is not possible in some circumstances, such as when Velero is included in a product.

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• Update v1.4 doc
• Update v1.5 doc
• Update v1.6 doc
• Test upgrade to v1.6.0

[carlisia]

@vmware-tanzu/velero-owners + @jenting: closing this as it has all been done as per #3623 and coordination.

[carlisia]

The documentation update was merged with this PR: #3568.