Alerting: Use time_intervals instead of the deprecated mute_time_intervals in a…

[soniaAguilarPeiron]

What is this feature?

This PR updates the way we use time intervals , after the the deprecation of mute_time_intervals from the alert manager config.

In this PR:

Grafana Alertmanager

Use mute_time_intervals and time_intervals when showing the UI, and save only on mute_time_intervals

External Alertmanager

• Combine both mute_time_intervals and time_intervals when showing the UI.
• Use time_intervals when creating new time intervals in the UI.
• When editing an existing time interval, if it came from a mute_time_interval save it back as a mute_time_interval, and if it came from a time_interval save it back as a time_interval

Fixes part of this issue

Why do we need this feature?

#83168

Who is this feature for?

All users.

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[soniaAguilarPeiron]

/deploy-to-hg

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[grobinson-grafana]

I think I found a bug. If there is mixed use of time_intervals and mute_time_intervals (for example, because time_intervals were created in the UI, but there are still provisioned mute_time_intervals), then the time_intervals are not shown until all mute_time_intervals are deleted:

[ephemeral-instances-bot]

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[soniaAguilarPeiron]

I think I found a bug. If there is mixed use of time_intervals and mute_time_intervals (for example, because time_intervals were created in the UI, but there are still provisioned mute_time_intervals), then the time_intervals are not shown until all mute_time_intervals are deleted:

good catch! I will update the pr with this use case.

[grobinson-grafana]

I tested it and LGTM! I'll let @gillesdemey and @konrad147 do the code review and approval.