botan: version 3 requires newer GCC (#39450)

e.g. 3.1.1 produces this during configuration when trying to install:
  ERROR: This version of Botan requires at least gcc 11.0

var/spack/repos/builtin/packages/botan/package.py

@@ -43,6 +43,8 @@ class Botan(MakefilePackage):
     depends_on("python", type="build")
     depends_on("py-sphinx@1.2:", type="build", when="+doc")
 
+    conflicts("%gcc@:10", when="@3:")
+
     def edit(self, spec, prefix):
         configure = Executable("./configure.py")
         configure(*self.configure_args())