feat(bigquery/storage): remove alpha client

[codyoss]

This api version should no longer be used.

Updates: #4098

[codyoss]

Created this as a feat so it shows up in release notes

[shollyman]

Seems there's still something related to internal snippets generation?

go: finding module for package cloud.google.com/go/bigquery/storage/apiv1alpha2
cloud.google.com/go/internal/generated/bigquery/storage/apiv1alpha2/BigQueryWriteClient/AppendRows imports
	cloud.google.com/go/bigquery/storage/apiv1alpha2: module cloud.google.com/go/bigquery@latest found (v1.18.0, replaced by ../../../bigquery), but does not contain package cloud.google.com/go/bigquery/storage/apiv1alpha2

[snippet-bot]

Here is the summary of changes.

You are about to delete 6 region tags.

• internal/generated/snippets/bigquery/storage/apiv1alpha2/BigQueryWriteClient/AppendRows/main.go:15, tag bigquerystorage_v1alpha2_generated_BigQueryWrite_AppendRows_sync
• internal/generated/snippets/bigquery/storage/apiv1alpha2/BigQueryWriteClient/BatchCommitWriteStreams/main.go:15, tag bigquerystorage_v1alpha2_generated_BigQueryWrite_BatchCommitWriteStreams_sync
• internal/generated/snippets/bigquery/storage/apiv1alpha2/BigQueryWriteClient/CreateWriteStream/main.go:15, tag bigquerystorage_v1alpha2_generated_BigQueryWrite_CreateWriteStream_sync
• internal/generated/snippets/bigquery/storage/apiv1alpha2/BigQueryWriteClient/FinalizeWriteStream/main.go:15, tag bigquerystorage_v1alpha2_generated_BigQueryWrite_FinalizeWriteStream_sync
• internal/generated/snippets/bigquery/storage/apiv1alpha2/BigQueryWriteClient/FlushRows/main.go:15, tag bigquerystorage_v1alpha2_generated_BigQueryWrite_FlushRows_sync
• internal/generated/snippets/bigquery/storage/apiv1alpha2/BigQueryWriteClient/GetWriteStream/main.go:15, tag bigquerystorage_v1alpha2_generated_BigQueryWrite_GetWriteStream_sync

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