Mailgun HTTP webhook signing key mislabelled as API Key

[ccalbreath]

Describe the Bug:
API Key should read HTTP webhook signing key

Steps to Reproduce:

1. Go to Admin->Config->Mailgun

Expected Behavior:
API Key should read HTTP webhook signing key so users enter the correct value from Mailgun.

Application Version:
Output of goalert version and/or version information from view-source on the UI.

/ # goalert version
Version: v0.32.0
GitCommit: 33ccf5e (clean)
BuildDate: 2024-03-12T19:39:26Z
GoVersion: go1.22.1 (gc)
Platform: linux/amd64
Migration: hb-addl-details (#242)

Additional Context:
I tried using the API Key from Mailgun but would get "first record does not look like a TLS handshake" when a new email was sent to the service Integration Key email address. Changing API Key to the value for HTTP Webhook signing key resolved the problem.