Upgrade to newer alertmanager dependency to remediate GHSA-v86x-5fm3-5p7j

[mamccorm]

The latest release of promxy is vulnerable to GHSA-v86x-5fm3-5p7j, due to a dependency on an old version of alertmanager:

promxy/go.mod

Line 129 in c6ce66d

github.com/prometheus/alertmanager v0.24.0 // indirect

Attempted bumping this to the minimum version required to address this CVE (v0.25.1), but results in the following build errors:

vendor/github.com/prometheus/prometheus/web/web.go:617:49: cannot use webConfig (variable of type string) as *"github.com/prometheus/exporter-toolkit/web".FlagConfig value in argument to toolkit_web.Serve

Not quite sure how to remediate this one - may require code changes or bumping other dependencies?