client/container: root: migrate to Botan3

- This is possible now that `dfi`'s ROOT build supports C++20
- No internal `dfi` API or impl changes are necessary
  * This includes any Botan signatures

container/src/root/macro/rootlogon.C

@@ -180,8 +180,7 @@ void help()
 void rootlogon()
 {
   // Add nested directory headers
-  // NOTE: Botan-3 is C++20 compliant (but ROOT isn't)
-  gInterpreter->AddIncludePath("/usr/include/botan-2");
+  gInterpreter->AddIncludePath("/usr/include/botan-3");
 
   // Link libraries
   gSystem->AddLinkedLibs("-lgtest");  // gtest/gmock