Update Prometheus + Alertmanager: Unify scripts for easier maintenance

[andygrunwald]

✍️ Description

Recently, Prometheus Alertmanager was merged, see #1272

Alertmanager is very similar in the installation as Prometheus.
This Pull Request unifies the scripts for Prometheus and Alertmanager, based of the feedback from #1272.

In particular:

• Using absolute pathes
• Prefixing ${RELEASE} with v
• Running update functions in /opt and not in ~
• Fixing a bug in the Alertmanager Update script (the rm -rf should fail, as we (originally) cded into the unpacked folder, but did not cd ..`)

For Prometheus, there is no functional change.
This is just a unification of both scripts.

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🛠️ Type of Change

• Bug fix (non-breaking change that resolves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change unexpectedly)
• New script (a fully functional and thoroughly tested script or set of scripts)

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✅ Prerequisites

• Self-review performed (I have reviewed my code to ensure it follows established patterns and conventions.)
• Testing performed (I have thoroughly tested my changes and verified expected functionality.)
• Documentation updated (I have updated any relevant documentation)

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📋 Additional Information (optional)

None

[MickLesk]

??? ^^

[andygrunwald]

@MickLesk May i ask why the ????

This PR is not about merging both script to one.
This PR is about to unify both scripts to work in the exact same way.

[MickLesk]

ok, lgtm, please test both scripts after merge

[andygrunwald]

Alertmanager

1. I installed Alertmanager (via bash -c "$(wget -qLO - https://github.com/community-scripts/ProxmoxVE/raw/main/ct/prometheus-alertmanager.sh)")
2. I downgraded manually to v0.26.0 (and verified it via http://192.168.178.38:9093/#/status)
3. I hit update in the console and followed the instructions
4. I verified the new version in http://192.168.178.38:9093/#/status and saw v0.27.0
5. I verified that there are no leftovers in /opt

Prometheus

The installation is failing with

A fix is proposed here: #1416

[andygrunwald]

After #1416 has been merged, here are the testing results for Prometheus

Prometheus

1. I installed Prometheus (via bash -c "$(wget -qLO - https://github.com/community-scripts/ProxmoxVE/raw/main/ct/prometheus.sh)")
2. I downgraded manually to v3.0.1 (and verified it via http://192.168.178.163:9090/status)
3. I hit update in the console and followed the instructions
4. I verified the new version in http://192.168.178.163:9090/status and saw v3.1.0
5. I verified that there are no leftovers in /opt