[QUAR-706][BpkPopover & BpkInsetBanner] Add custom z-index value

[kirstybryce]

JIRA: https://skyscanner.atlassian.net/browse/QUAR-706
Thread with context: https://skyscanner.slack.com/archives/C0JHPDSSU/p1740574285075669

When trying to use the BpkInsetBanner with CTA popover in the Banana booking panel page, there were 2 other backpack components on the page with higher z-index taking priority and preventing the popover of the inset banner from showing. To fix this we decided to add a custom z-index prop (zIndexCustom ) to the inset banner component that gets passed to the popover component (via zIndexValue prop). If a z-index value is provided then that can be used for the popover, otherwise the default z-index of 900 is used.

Before (default z-index: 900)	After (custom z-index: 1100)

Remember to include the following changes:

• Ensure the PR title includes the name of the component you are changing so it's clear in the release notes for consumers of the changes in the version e.g [KOA-123][BpkButton] Updating the colour
• README.md (If you have created a new component)
• Component README.md
• Tests
• Accessibility tests
  • The following checks were performed:
    • Ability to navigate using a keyboard only
    • Zoom functionality (Deque University explanation):
      • The page SHOULD be functional AND readable when only the text is magnified to 200% of its initial size
      • Pages must reflow as zoom increases up to 400% so that content continues to be presented in only one column i.e. Content MUST NOT require scrolling in two directions (both vertically and horizontally)
    • Ability to navigate using a screen reader only
• Storybook examples created/updated
• For breaking changes or deprecating components/properties, migration guides added to the description of the PR. If the guide has large changes, consider creating a new Markdown page inside the component's docs folder and link it here

[skyscanner-backpack-bot]

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[skyscanner-backpack-bot]

	Warnings
⚠️	Package source files (e.g. packages/package-name/src/Component.js) were updated, but snapshots weren't. Have you checked that the tests still pass?
⚠️	Package source files (e.g. packages/package-name/src/Component.tsx) were updated, but type files weren't. Have you checked that no types have changed?

Browser support

If this is a visual change, make sure you've tested it in multiple browsers.

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[skyscanner-backpack-bot]

Visit https://backpack.github.io/storybook-prs/3763 to see this build running in a browser.