Break down the Serving and net-istio artifact downloads into their own release trains

[matzew]

What this PR does / why we need it:

Knative-Serving and Knative's Net-Istio are on a different, independent release train, reflecting this in the quick installer.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Type of changes
Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

More an enhancement, than a bug fix

Feature/Issue validation/testing:

Changes:

• introduced new ENV_VARs for Knative_Serving and Knative_Istio, since they have indendent release trains.

[x] Tested by running the quickstart against KinD locally, after the changes

• Logs

Special notes for your reviewer:

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Checklist:

• Have you added unit/e2e tests that prove your fix is effective or that this feature works?
• Has code been commented, particularly in hard-to-understand areas?
• Have you made corresponding changes to the documentation?

Release note:

[matzew]

CC @aliok

[yuzisun]

Thanks @matzew !

/lgtm
/approve

[kserve-oss-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: matzew, yuzisun

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