GA: Promote `Receiver` to `notification.toolkit.fluxcd.io/v1`

[stefanprodan]

Items to be reviewed plus any outstanding work required to bump the API to v1:

• Documentation
  • https://fluxcd.io/flux/guides/webhook-receivers/
  • https://fluxcd.io/flux/guides/image-update/#trigger-image-updates-with-webhooks
  • https://fluxcd.io/flux/guides/helmreleases/#configure-webhook-receivers
  • https://fluxcd.io/flux/migration/helm-operator-migration/#added-support-for-notifications-and-webhooks
  • https://fluxcd.io/flux/components/notification/options/#flags
  • https://fluxcd.io/flux/components/notification/receiver/
  • https://fluxcd.io/flux/components/notification/api/#notification.toolkit.fluxcd.io/v1beta2.Receiver
  • https://fluxcd.io/flux/gitops-toolkit/packages/#go-packages
  • https://fluxcd.io/flux/flux-e2e/#trigger-reconciling-on-git-push-with-webhook-receivers
  • https://fluxcd.io/flux/cheatsheets/bootstrap/#enable-notifications-for-third-party-controllers
• API Validation
  • all Receiver API fields are accompanied with proper validation if possible
  • the validation is reflected in the documentation, particuarly https://fluxcd.io/flux/components/notification/receiver/.
• Test Coverage
  • ReceiverReconciler: 94.9% direct coverage
• GA: Promote Receiver API to notification.toolkit.fluxcd.io/v1 #498
• Move controllers into internal package #499
• Release 1.0.0-rc.1 #503
• tag v1 release

Depends on: #435