Properly validate milestones array in releases API PUT method

What does this MR do?

Discovered during debugging of #277369 (closed). We already have this validation for POST method. But not for PUT.

Currently, the random data can cause problems like #277369 (comment 443924350). So we want to validate that this array is array of strings.

It won't fix the frontend bug, but at least API will return 4** and not 500 with a meaningful message.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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