feat(alerting): add timezone for maintenance

[lefes]

Summary

Add a "Timezone" field to the configuration in the maintenance zone to specify the timezone for the maintenance window. Also, include tests for various timezones and cases.

Fixes #327

Checklist

• [X] Tested and/or added tests to validate that the changes work as intended, if applicable.
• [X] Updated documentation in README.md, if applicable.

[lefes]

@TwiN Can you review this PR? I've made some changes that you are asked for

[TwiN]

@lefes Thank you for your contribution! 🙇