ci(tags): lowercase image tags (backport #9)

[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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Fixes: #8

Test run: https://github.com/andrewazores/test-applications/actions/runs/9893879116/job/27330155145

Results:

• https://quay.io/repository/andrewazores/gameserver-cryostat-agent?tab=tags
• https://quay.io/repository/andrewazores/quarkus-cryostat-agent?tab=tags
• https://quay.io/repository/andrewazores/vertx-cryostat-agent?tab=tags

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  This is an automatic backport of pull request #9 done by Mergify.