doc: updated README.md by removing the vpn gateway references.

[imprateeksh]

Description

Update the Readme doc to remove the VPN information which is not valid.

Issue reference - #921

Release required?

• No release
• Patch release (x.x.X)
• Minor release (x.X.x)
• Major release (X.x.x)

Release notes content

Run the pipeline

If the CI pipeline doesn't run when you create the PR, the PR requires a user with GitHub collaborators access to run the pipeline.

Run the CI pipeline when the PR is ready for review and you expect tests to pass. Add a comment to the PR with the following text:

/run pipeline

Checklist for reviewers

• If relevant, a test for the change is included or updated with this PR.
• If relevant, documentation for the change is included or updated with this PR.

For mergers

• Use a conventional commit message to set the release level. Follow the guidelines.
• Include information that users need to know about the PR in the commit message. The commit message becomes part of the GitHub release notes.
• Use the Squash and merge option.

[maheshwarishikha]

@imprateeksh VPN gateway support is about to come in via my VPC-DA PR. PR is under review.
Would be helpful if you can hold your changes till then and then update whatever is not required. Thanks.

[imprateeksh]

Closing this PR as the issue is no longer relevant. VPN gateway references are now part of terraform-ibm-landing-zone-vpc