[server] Add /ready probe to smooth rollout of server pods

[geropl]

Description

This PR does two things:

1. Rename ReadinessController to StartupController and mount it on /startup, because it's used by the StartupProbe
2. Introduce a fresh ReadinessController which:
   • always returns 👍
   • except when we already received SIGTERM, switching to 👎

Related Issue(s)

Fixes CLC-1234

How to test

• installation: https://gpl-1234-sda36a70e7c.preview.gitpod-dev.com/workspaces
• open a workspace on this PR
• do theses changes on the server deployment:
  • scale server up and down
  • edit and add/remove/change an env var
• and notice how you no longer see 2/2 Terminating but 0/2 Terminating instead ✔️

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
• /werft preemptible
  Saves cost. Untick this only if you're really sure you need a non-preemtible machine.
• 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

[corneliusludmann]

👍

[geropl]

/unhold