fix(GlTooltip): Fix tooltip hide/show animation

What does this MR do?

I’m working on rolling out the GitLab UI tooltips in the application, and I noticed that tooltips does not have a hide/show animation anymore. I tried disabling this opacity change locally, and I noticed that animations were back:

Before fix	After fix

I looked at the BVTooltip’s source code, and I noticed that the visible opacity is applied to a different class. This Merge Request moves the opacity change to the show class.

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.
• Added the ~"component:*" label(s) if applicable.