chore(all): auto-regenerate .pb.go files (#779)

This is an auto-generated regeneration of the .pb.go files by
cloud.google.com/go/internal/gapicgen. Once this PR is submitted, genbot will
update the corresponding PR to depend on the newer version of go-genproto, and
assign reviewers. Whilst this or any regen PR is open in go-genproto, genbot
will not create any more regeneration PRs. If all regen PRs are closed,
gapicgen will create a new set of regeneration PRs once per night.

If you have been assigned to review this PR, please:

- Ensure that CI is passing. If it's failing, it requires your manual attention.
- Approve and submit this PR if you believe it's ready to ship. That will prompt
genbot to assign reviewers to the google-cloud-go PR.

Corresponding google-cloud-go PR: googleapis/google-cloud-go#5767

Changes:

chore(spanner/admin/database): remove unused imports from google/spanner/*
  PiperOrigin-RevId: 435163448
  Source-Link: googleapis/googleapis@db9d2a3

feat: add `content` field in TextAnchor
  PiperOrigin-RevId: 435142243
  Source-Link: googleapis/googleapis@721b367

chore: regenerate API index

  Source-Link: googleapis/googleapis@6c79119

feat: Publish Cloud Batch v1alpha1 API Client
  PiperOrigin-RevId: 435137708
  Source-Link: googleapis/googleapis@9c07d5f

feat(aiplatform): add `service_account` to `BatchPredictionJob` in aiplatform `v1beta1` `batch_prediction_job.proto`
  PiperOrigin-RevId: 434935416
  Source-Link: googleapis/googleapis@dfdd5ee

feat(dataflow): Add the ability to plumb environment capabilities through v1beta3 protos.
  PiperOrigin-RevId: 434899727
  Source-Link: googleapis/googleapis@392ae91