[ARGG-1159][BpkModal]: Remove portal from modal

[olliecurtis]

Similar to #3508

Migrates the BpkModal to not use the custom portal implementation from Backpack.

All functionality has been retained, the only main change here is remove the use of BpkPortal and replace the state management to be local to the component.

Remember to include the following changes:

• [x] 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
• [x] Tests
• [x] Accessibility tests
  • The following checks were performed:
    • [x] Ability to navigate using a keyboard only
    • [x] Zoom functionality (Deque University explanation):
      • [x] The page SHOULD be functional AND readable when only the text is magnified to 200% of its initial size
      • [x] 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)
    • [x] Ability to navigate using a screen reader only
• [x] 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