Add namespace template variable

[fgeck]

Description of the change

Added namespaceOverride template variable to support flexible namespace configurations.
Introduced helper function in helpers.tpl for namespace resolution.
Updated relevant templates to utilize the new namespace logic.
Bumped Helm chart version to 2.21.2.

Benefits

A user can deploy the uptime-kuma helm chart in a configured namesapce when deploying.

Possible drawbacks

None that come to mind.

Applicable issues

N/A

Additional information

• all credits go to @asemarian, I just addressed the review of @dirsigler that he is able to merge the PR
• /closes Add namespace template variable #181

Checklist

• Chart version bumped in Chart.yaml according to semver.
• Variables are documented in the README.md

[fgeck]

Lint and test issues seem not related to my changes:

Version 3.7 was not found in the local cache
Error: The version '3.7' with architecture 'x64' was not found for Ubuntu 24.04.
The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

[dirsigler]

Updated some version and pushed into main.
Please rebase from main, it should work afterwards.

[fgeck]

@dirsigler just did a merge via github ui instead of a rebase, hope it is fine when you do a squash merge when merging into main branch.