[Backport] Update Envoy Gateway to v1.9.1 in 10.3

What does this MR do?

Backports minor Envoy bump Update Envoy Gateway to v1.9.1 (!5300 - merged) (landed in 19.4) to 10.3. Also updating relevant entries of Envoy version in docs to be up-to-date

Per request from Dedicated team !5300 (comment 3880178393)

Diff https://github.com/envoyproxy/gateway/compare/v1.9.0...v1.9.1

Related issues

!5300 (comment 3880178393)

Author checklist

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• Documentation created/updated. - n/a
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  • Local deployment and CI jobs. Please also see !5381 (comment 3883905739)

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