chore(release): prepare for 3.0.0 release

[andrewazores]

• ci(tags): tag images as 'latest'
• test(smoketest): run 'latest' Cryostat image tag
• chore(release): enable mergify backport rule
• chore(release): update project to 4.0.0-SNAPSHOT, use -core 4.0.0-SNAPSHOT

Welcome to Cryostat3! 👋

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

[andrewazores]

/build_test

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[github-actions]

CI build and push: All tests pass ✅ (JDK17)
https://github.com/cryostatio/cryostat3/actions/runs/9390164387