update Nokia logo in case-studies

[nate-double-u]

Description

Nokia has asked that we change their logo on the case studies pages.

• https://kubernetes.io/case-studies/
• https://kubernetes.io/case-studies/nokia/

I've also changed or removed the background banners on the Nokia case-studies page as they had the old logos in as well.

[k8s-ci-robot]

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[dipesh-rawat]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: a43e1dbaf9f4afc286732404eedd3eff51f74715

[Arhell]

lgtm