web/admin: update flow background (#11758)

* web/admin: update flow background

https://unsplash.com/photos/gray-concrete-road-between-trees-near-mountain-z8ct_Q3oCqM
Signed-off-by: Jens Langhammer <jens@goauthentik.io>

* Optimised images with calibre/image-actions

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Signed-off-by: Jens Langhammer <jens@goauthentik.io>
Co-authored-by: authentik-automation[bot] <135050075+authentik-automation[bot]@users.noreply.github.com>