fix: set `display: flex` for sidenav by default

[ersachin3112]

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What kind of change does this PR introduce?

[x] Bugfix
[ ] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
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What is the current behavior?

Fixes #10040

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Does this PR introduce a breaking change?

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No

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[ersachin3112]

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[Splaktar]

This is likely not something that we can get merged during the LTS. It would have been possible to consider it last July for the 1.2.0 release since this change is likely to break some apps.

For now, it's probably best to just override this style in your app.