Alerting: Fix firing alerts title when showing active in Insights panel

[soniaAguilarPeiron]

What is this feature?

This PR fixes Insights panel title for showing firing alerts count.
So far, this panel was counting alerts with 'active' state when an active rule is not always firing, so the number is incorrect.

After reviewing the rest of the panels, we realised that actually there is another panel showing the firing count. So, we think that the problem here is not the count but the title, as there is no other panel showing active alerts.

Who is this feature for?

All cloud users.

Which issue(s) does this PR fix?:

Fixes #80348

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.

[gillesdemey]

Changes LGTM!

[soniaAguilarPeiron]

/deploy-to-hg

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