Suggest cargo install on pgrx version mismatch diagnostic (#1579)

Resolves #1444

cargo-pgrx/src/metadata.rs

@@ -68,11 +68,13 @@ pub fn validate(
         return Err(eyre!(
             "The installed `cargo-pgrx` v{cargo_pgrx_version} \
             is not compatible with the {mismatches} {} in `{}`. `cargo-pgrx` \
-            and pgrx dependency versions must be identical.",
+            and pgrx dependency versions must be identical.
+            {help} cargo install --version {mismatches} --locked",
             if many == 1 { "dependency" } else { "dependencies" },
             path.map(|p| p.as_ref().display().to_string())
                 .unwrap_or_else(|| "./Cargo.toml".to_string())
-                .yellow()
+                .yellow(),
+            help = "help:".to_string().bold()
         ));
     }