fix: Make selector more robust against other siblings and fix storybook

What does this MR do?

Fixes #1732 (closed) by using the ~ selector instead of + to make it more robust against other sibling elements sitting between the .form-control and the element itself. This happens when the BFormGroup component's state is valid (true) while at the same time having a invalidFeedback text. In such a case, the .invalid-feedback div is only set to display: none instead of removed from the DOM.

Update: During review we found out that the initial approach didn't exactly work. See details below. So instead of changing from + to ~ I've added an extra selector specifically for that one inconsistent state described above.

We noticed this behavior first in the storybook where we had a small > vs >= bug, which this MR also fixes.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.