Bug 1715019 - Update README.botan for version 2.17.3. r=kaie

Differential Revision: https://phabricator.services.mozilla.com/D117251

--HG--
extra : moz-landing-system : lando

third_party/README.botan

@@ -1,10 +1,10 @@
-Directory ./botan contains a copy of version 2.15.0 of the Botan library,
-which has been obtained from https://botan.randombit.net/releases/Botan-2.15.0.tar.xz .
+Directory ./botan contains a copy of version 2.17.3 of the Botan library,
+which has been obtained from https://botan.randombit.net/releases/Botan-2.17.3.tar.xz .
 
 For licensing information, please refer to the included documentation.
 
 The SHA256SUM of the imported file is:
-d88af1307f1fefac79aa4f2f524699478d69ce15a857cf2d0a90ac6bf2a50009  Botan-2.15.0.tar.xz
+79123b654445a4abba486e09a431788545c708237382a3e765664c9f55b03b88  Botan-2.17.3.tar.xz
 
 The following files were removed from the source distribution:
 .gitignore