Fix upgrade check to allow upgrades to 2.x.x

[tenczar]

This change updates are upgrade validation logic to allow users to upgrade from 1.6.x to 2.x.x. It will return an error if a user attempts to upgrade from a version less than 1.6 or if they try to upgrade to major version 3. Obviously that check will need to be updated on release of major version 3.

What this PR does / why we need it

This fix allows us to move to TKG with major version 2. A previous check would throw an error if the major version was changed.

Which issue(s) this PR fixes

Fixes #

Describe testing done for PR

Updated unit tests to include validation that upgrades to major version 3 still throw an error. Also added test to make sure an error is thrown when upgrade to v2 from a version less than v1.6

Release note

Upgrades to major version 2 no longer throw an error.

Additional information

Special notes for your reviewer

[github-actions]

Cluster Generation A/B Results:
https://storage.googleapis.com/tkg-clustergen/3990/20221123203036/clustergen.diff.txt
Author/reviewers:
Please review to verify that the effects on the generated cluster configurations are exactly what the PR intended, and give a thumbs-up if so.

[codecov]

Codecov Report

Merging #3990 (c0f742c) into main (69bb600) will decrease coverage by 0.86%.
The diff coverage is 26.31%.

@@            Coverage Diff             @@
##             main    #3990      +/-   ##
==========================================
- Coverage   48.22%   47.36%   -0.87%     
==========================================
  Files         433      456      +23     
  Lines       43122    44597    +1475     
==========================================
+ Hits        20795    21122     +327     
- Misses      20343    21469    +1126     
- Partials     1984     2006      +22     

Impacted Files	Coverage Δ
tkg/client/client.go	81.25% <ø> (ø)
tkg/client/init.go	4.87% <0.00%> (-0.08%)	⬇️
tkg/clusterclient/clusterclient.go	48.20% <0.00%> (-0.08%)	⬇️
tkg/client/upgrade_region.go	23.70% <100.00%> (+0.35%)	⬆️
addons/controllers/clusterbootstrap_controller.go	64.75% <0.00%> (-1.64%)	⬇️
...ons/controllers/packageinstallstatus_controller.go	79.15% <0.00%> (-1.16%)	⬇️
...i/plugin/cluster/delete_machinehealthcheck_node.go	16.66% <0.00%> (ø)
cmd/cli/plugin/cluster/credentials_update.go	8.73% <0.00%> (ø)
... and 22 more

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[github-actions]

Cluster Generation A/B Results:
https://storage.googleapis.com/tkg-clustergen/3990/20221130011038/clustergen.diff.txt
Author/reviewers:
Please review to verify that the effects on the generated cluster configurations are exactly what the PR intended, and give a thumbs-up if so.

[zjs]

LGTM