README: Update logo

Introduce logo for dark version of github

Author: hynekkar

README.md

@@ -1,6 +1,9 @@
-<p align="center">
-    <img src="https://raw.githubusercontent.com/CESNET/ipfixprobe/refs/heads/master/docs/images/ipfixprobe-horizontal.svg" width="450">
-</p>
+<div align="center">
+<picture>
+  <source srcset="https://raw.githubusercontent.com/CESNET/ipfixprobe/refs/heads/master/docs/images/logo/logo_horizontal_white.svg" width="450" media="(prefers-color-scheme: dark)">
+  <img src="https://raw.githubusercontent.com/CESNET/ipfixprobe/refs/heads/master/docs/images/logo/logo_horizontal_color.svg" width="450">
+</picture>
+</div>
 
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