[Direct PR] [release-18.0]: Add sidecardb known issue

[GuptaManan100]

Description

This PR adds the known issue with sidecard db where it continues to find a schema diff and applies the change that doesn't change the DDL. Caused when #15859 was merged.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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[vitess-bot]

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[frouioui]

It seems like this was fixed on v18.0.7, if that is the case, we should update the release notes from 18.0.0 to 18.0.6 included.

Moreover, we should change the release_notes.md file directly, so that we just have to copy paste it into the GH UI once merged.

[deepthi]

With comments 😀