fix(GlPopover): Fix custom triggers not working due to hard-coded triggers

What does this MR do?

A popover's default triggers property is click. In a previous MR this was changed to click focus, but it was inadvertently hard-coded and could not be overridden. This breaks a number of components in the GitLab project that uses custom triggers, and this MR fixes it so that custom triggers can be passed in.

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.

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.