Update otelopscol submodule to v0.156.0

[rafaelwestphal]

Description

Also update the github.com/GoogleCloudPlatform/opentelemetry-operations-collector/integration_test/gce-testing-internal dependency to match the new submodule version.

Related issue

How has this been tested?

Checklist:

• Unit tests
  • Unit tests do not apply.
  • Unit tests have been added/modified and passed for this PR.
• Integration tests
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  • Integration tests have been added/modified and passed for this PR.
• Documentation
  • This PR introduces no user visible changes.
  • This PR introduces user visible changes and the corresponding documentation change has been made.
• Minor version bump
  • This PR introduces no new features.
  • This PR introduces new features, and there is a separate PR to bump the minor version since the last release already.
  • This PR bumps the version.