Alerting: Enable simplified routing FF by default

[JacobsonMT]

What is this feature?

Moves simplified routing feature flag alertingSimplifiedRouting to GA and enables by default. Removes admonitions from docs about the FF.

Why do we need this feature?

Simplified routing should be GA and enabled by default in G11.

Who is this feature for?

Grafana Alerting users.

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.

[yuri-tceretian]

perhaps move it to another PR?

[JacobsonMT]

The test will fail without this change, I could perhaps disable the simplfied routing FF here instead but fixing the test seems best.

[brendamuir]

Hey Matt -- so do we only need to keep this section in the docs then? https://grafana.com/docs/grafana-cloud/alerting-and-irm/alerting/alerting-rules/create-grafana-managed-rule/#configure-labels-and-notifications and can i delete this section (that describes the process when simplified is NOT enabled): https://grafana.com/docs/grafana-cloud/alerting-and-irm/alerting/alerting-rules/create-grafana-managed-rule/#configure-notifications ? if so, lets also delete that section with this PR :)

[JacobsonMT]

Hey Matt -- so do we only need to keep this section in the docs then? https://grafana.com/docs/grafana-cloud/alerting-and-irm/alerting/alerting-rules/create-grafana-managed-rule/#configure-labels-and-notifications and can i delete this section (that describes the process when simplified is NOT enabled): https://grafana.com/docs/grafana-cloud/alerting-and-irm/alerting/alerting-rules/create-grafana-managed-rule/#configure-notifications ? if so, lets also delete that section with this PR :)

Ah, nice catch! Didn't realize the section was duplicated. Yes, I'll make the change in this PR.