Always generate unique name for `DastSiteProfile` in specs

What does this MR do?

This MR fixes the flaky specs for the DastSiteProfile model. Seems like there was an implicit assumption that the FFaker gem always generates unique values which is not correct so using the values produced by the gem for an attribute with uniqueness validation led to random failure in pipelines. Here is one of the cases: https://gitlab.com/gitlab-org/gitlab/-/jobs/922588822

I've also added truncation logic to truncate the sample data just in case if somehow we get one longer than 255 chars which would break the length validation.

We could also use the unique feature of FFaker gem but it has a limit(can be increased though) and it stores all the values generated in memory which will increase the memory consumption and slow down the test suite.

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

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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