docs(readme): update readme link for old repo (backport #534)

[mergify[bot]]

Welcome to Cryostat! 👋

Before contributing, make sure you have:

• [x] Read the contributing guidelines
• [x] Linked a relevant issue which this PR resolves
• [x] Linked any other relevant issues, PR's, or documentation, if any
• [x] Resolved all conflicts, if any
• [x] Rebased your branch PR on top of the latest upstream main branch
• [x] Attached at least one of the following labels to the PR: [chore, ci, docs, feat, fix, test]
• [x] Signed all commits using a GPG signature

To recreate commits with GPG signature git fetch upstream && git rebase --force --gpg-sign upstream/main

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Related to #469

Description of the change:

This change allows an environment variable to be configured so that...

Motivation for the change:

This change is helpful because users may want to...

How to manually test:

1. Run CRYOSTAT_IMAGE=quay.io... bash smoketest.bash...
2. ...

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   This is an automatic backport of pull request #534 done by Mergify.