Ensure VSM stage has relative position

What does this MR do?

Ensure that all VSM stage has relative position before moving a stage. This fixes a problem where moving a stage after another stage (where the relative position was null) sets the position incorrectly.

• Problem was reported here: !26074 (comment 308687856)
• Related issue: #196698 (closed)

How to reproduce:

1. Go to group VSM.
2. Add a stage.
3. Add another stage.
4. Move "Test" stage to the bottom.
5. Reload the page
6. "Test" stage is not at the bottom.

After this MR:

"Test" stage should be under the bottom.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
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• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
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