fix(components/modals): set modal content tabindex to make scrollable area focusable (#619)

[Blackbaud-ErikaMcVey]

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✅ Successfully ran 18 targets

• nx affected --target=build --parallel=3
• nx affected --target=test --browsers=ChromeHeadless --code-coverage --parallel=3 --progress=false --source-map --watch=false
• nx affected:lint --quiet --silent --parallel=3
• nx build code-examples --baseHref=https://blackbaud.github.io/skyux-pr-preview/625/code-examples/
• nx build integration --baseHref=https://blackbaud.github.io/skyux-pr-preview/625/integration/
• nx build playground --baseHref=https://blackbaud.github.io/skyux-pr-preview/625/playground/
• nx e2e ag-grid-storybook-e2e --skip-nx-cache
• nx e2e layout-storybook-e2e --skip-nx-cache
• nx e2e modals-storybook-e2e --skip-nx-cache
• nx e2e pages-storybook-e2e --skip-nx-cache
• nx e2e tabs-storybook-e2e --skip-nx-cache
• nx run ag-grid-storybook:build-storybook:ci
• nx run i18n:postbuild
• nx run layout-storybook:build-storybook:ci
• nx run modals-storybook:build-storybook:ci
• nx run pages-storybook:build-storybook:ci
• nx run storybook:build-storybook:ci --outputDir=dist/storybook
• nx run tabs-storybook:build-storybook:ci

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• modals-storybook
• pages-storybook
• tabs-storybook

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• integration
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