fix: using UTC timezone for alert levels

[vlastahajek]

Required checklist

• Sample config files updated (both /etc folder and NewDemoConfig methods) (influxdb and plutonium)
• openapi swagger.yml updated (if modified API) - link openapi PR
• Signed CLA (if not already signed)

Description

Forcing batch alert time timezone to UTC. Fixes #938 and EAR #5376
Stream alert also has the time in the UTC zone.

Context

Affected areas (if applicable):

The .Time variable printed as a string will always be in UTC.

Severity (optional)

Note for reviewers:

Check the semantic commit type:

• Feat: a feature with user-visible changes
• Fix: a bug fix that we might tell a user “upgrade to get this fix for your issue”
• Chore: version bumps, internal doc (e.g. README) changes, code comment updates, code formatting fixes… must not be user facing (except dependency version changes)
• Build: build script changes, CI config changes, build tool updates
• Refactor: non-user-visible refactoring
• Check the PR title: we should be able to put this as a one-liner in the release notes

[alespour]

The fix makes sense. Consistency with stream alerts is a great outcome. Tests checks what is needed.

[karel-rehor]

Tests pass locally. Changes make sense.