Simplify rustc-check-cfg emission in build.rs

(backport <#4724>)
(cherry picked from commit f093797)

Author: bjorn3

build.rs

@@ -145,19 +145,11 @@ fn main() {
     // avoid warnings.
     if rustc_minor_ver >= 80 {
         for cfg in ALLOWED_CFGS {
-            if rustc_minor_ver >= 75 {
-                println!("cargo:rustc-check-cfg=cfg({cfg})");
-            } else {
-                println!("cargo:rustc-check-cfg=values({cfg})");
-            }
+            println!("cargo:rustc-check-cfg=cfg({cfg})");
         }
         for &(name, values) in CHECK_CFG_EXTRA {
             let values = values.join("\",\"");
-            if rustc_minor_ver >= 75 {
-                println!("cargo:rustc-check-cfg=cfg({name},values(\"{values}\"))");
-            } else {
-                println!("cargo:rustc-check-cfg=values({name},\"{values}\")");
-            }
+            println!("cargo:rustc-check-cfg=cfg({name},values(\"{values}\"))");
         }
     }
 }