[obs] remove GitpodImagebuildStartSuccess warning

[kylos101]

Description

This expression has dips regularly, and does not provide value as a notification in its current form.

Summary generated by Copilot

🤖 Generated by Copilot at f003f87

Removed an unreliable alert for image build start success and added a new alert for image build duration. This improves the observability of image build performance and user experience.

Related Issue(s)

Fixes n/a

How to test

n/a

Documentation

Preview status

gitpod:summary

Build Options

Build

• /werft with-werft
  Run the build with werft instead of GHA
• leeway-no-cache
• /werft no-test
  Run Leeway with --dont-test

Publish

• /werft publish-to-npm
• /werft publish-to-jb-marketplace

Installer

• analytics=segment
• with-dedicated-emulation
• workspace-feature-flags
  Add desired feature flags to the end of the line above, space separated

Preview Environment / Integration Tests

• /werft with-local-preview
  If enabled this will build install/preview
• /werft with-preview
• /werft with-large-vm
• /werft with-gce-vm
  If enabled this will create the environment on GCE infra
• with-integration-tests=all
  Valid options are all, workspace, webapp, ide, jetbrains, vscode, ssh. If enabled, with-preview and with-large-vm will be enabled.
• with-monitoring

/hold