fix(PageHeader): use display block insteaf of flex when FF off

[francinelucca]

Corrects migration error on https://github.com/primer/react/pull/5332/files#diff-3d5579394f58fc79a828428e1c64fb429de09d23d117a6ecd23875098d4eba86R540-R557

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Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

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🦋 Changeset detected

Latest commit: e293672

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@primer/react	Patch

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	102.32 KB (+0.03% 🔺)
packages/react/dist/browser.umd.js	102.81 KB (+0.11% 🔺)