docs: update banner img src
-<h1 align="center"><img src=".github/images/banner_hydra.png" alt="ORY Hydra - Open Source OAuth 2 and OpenID Connect server"></h1>
+<h1 align="center"><img src=".github/images/banner_hydra.svg" alt="ORY Hydra - Open Source OAuth 2 and OpenID Connect server"></h1>
<h4 align="center">
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