Remove Subscription v1alpha1 version

[muralov]

Description Delete the v1alpha1 version of Subscription API

Acceptance Criteria

• [x] Delete v1alpha1 Subscription CRD
• [x] Delete v1alpha1 code
• [x] Delete v1alpha1 webhooks
• [x] Verify that the upgrade to this version has no issue
• [x] Delete v1alpha1 E2E tests
• [ ] Check if any other components not using v1alpha1 related packages, for example EPP is using it here We decided to keep it as it is.
• [x] Verify the load tester that is not using v1alpha1
• [x] Delete internal and external documentations of v1alpha1

[mfaizanse]

Checklist for Delete internal and external documentations of v1alpha1:

• [x] ~On-call guides~: PR is added to draft, it will be removed once eventing-manager is released to prod.
• [x] SAP help
  • [x] https://help.sap.com/docs/btp/sap-business-technology-platform/choose-backend-for-kyma-eventing?q=kyma%20eventing
  • [x] https://help.sap.com/docs/btp/sap-business-technology-platform/using-kyma-eventing-with-sap-event-mesh?q=kyma%20eventing
• [x] EM repository docs
  • [x] https://kyma-project.io/#/eventing-manager/user/resources/evnt-cr-subscription?id=subscriptioneventingkyma-projectiov1alpha1

PR(s):

• [x] https://github.com/kyma-project/eventing-manager/pull/524

[mfaizanse]

Checklist for Verify the load tester that is not using v1alpha1:

• [x] For v1alpha1 we have a legacy branch.
• [x] Validation tests: link
  • [x] https://github.com/kyma-project/eventing-manager/pull/529
  • [x] https://github.com/kyma-project/eventing-tools/pull/96
• [x] Test load tests with v1alpha1 subscription version removed.