Remove Subscription v1alpha1 version

[muralov]

Description
Delete the v1alpha1 version of Subscription API

Acceptance Criteria

• Delete v1alpha1 Subscription CRD
• Delete v1alpha1 code
• Delete v1alpha1 webhooks
• Verify that the upgrade to this version has no issue
• 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.
• Verify the load tester that is not using v1alpha1
• Delete internal and external documentations of v1alpha1

[mfaizanse]

Checklist for Delete internal and external documentations of v1alpha1:

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

PR(s):

• chore: removed v1alpha1 subscription specifications from doc #524

[mfaizanse]

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

• For v1alpha1 we have a legacy branch.
• Validation tests: link
  • refactor: added event types in e2e tests #529
  • refactor: removed validation tests eventing-tools#96
• Test load tests with v1alpha1 subscription version removed.