[DCAAS] refactoring opentelekomcloud_dc_virtual_gateway_v2, deprecation opentelekomcloud_dc_endpoint_group_v2

[anton-sidelnikov]

Summary of the Pull Request

Impossible to properly use endpoint group separately from virtual gateway because of api structure, cyclic dependecies. Make decision to move endpoint group management under virtual gateway resource.
Added deprecation message to opentelekomcloud_dc_endpoint_group_v2

PR Checklist

• Refers to: opentelekomcloud_dc_virtual_interface_v2 fails to create virtual interface #2393
• Tests added/passed.
• Documentation updated.
• Schema updated.
• Release notes added.

Acceptance Steps Performed

=== RUN   TestDirectConnectVirtualGatewayV2Resource_basic
--- PASS: TestDirectConnectVirtualGatewayV2Resource_basic (82.87s)
PASS

Debugger finished with the exit code 0

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Build succeeded.
https://zuul.otc-service.com/t/eco/buildset/74fa0c5ab825479986572dab559f3e35

✔️ noop SUCCESS in 0s
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✔️ golang-make-test SUCCESS in 4m 27s
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