README.md: Update Clang/LLVM dependency to 12

Relying on Clang 1.12 doesn't make sense. Update it to 12.

Signed-off-by: Joe Stringer <joe@isovalent.com>

Author: joestringer

README.md

@@ -145,7 +145,7 @@ See [docs/vagrant.md](docs/vagrant.md)
 ### Dependencies
 
 * Go >= 1.16
-* LLVM/clang >= 1.12
+* LLVM/clang >= 12
 * Bison
 * Lex/Flex >= 2.5.31