Fix sticky local nav and breadcrumbs after global header update

[ryelle]

When WordPress/wporg-mu-plugins#466 merges, it will unstick the global header and only stick the local nav (and breadcrumbs). This will affect the way the local navigation bar stickiness works on the Developers site, because it uses the is-sticky helper class. Instead, it can now use "position":{"type":"sticky"} to use the gutenberg sticky behavior.

See WordPress/wporg-documentation-2022#71 for a similar fix on documentation.

[adamwoodnz]

Closed by #348