fix(release): added beta check and dummy version logics

[tomcat323]

Problem

in #6426, a new release pipeline stage was added that checks the marketplace extension has been updated. This PR addes a stage check and only look for an existing version of the extension when in BETA.

Output in beta:

unblocks: https://pipelines.amazon.com/pipelines/AWSVSCodeInfrastructure

[github-actions]

• This pull request implements a feat or fix, so it must include a changelog entry (unless the fix is for an unreleased feature). Review the changelog guidelines.
  • Note: beta or "experiment" features that have active users should announce fixes in the changelog.
  • If this is not a feature or fix, use an appropriate type from the title guidelines. For example, telemetry-only changes should use the telemetry type.