build-configs.yaml: move rust-for-linux_rust-next to Rust 1.71

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

config/core/build-configs.yaml

@@ -1414,8 +1414,8 @@ build_configs:
     tree: rust-for-linux
     branch: 'rust-next'
     variants:
-      rustc-1.70:
-        build_environment: rustc-1.70
+      rustc-1.71:
+        build_environment: rustc-1.71
         fragments: [rust, rust-samples, kselftest]
         architectures:
           x86_64: