fix(msrv): Error, rather than panic, on rust-version 'x'

Fixes #13768

Author: epage

crates/cargo-util-schemas/src/core/partial_version.rs

@@ -183,5 +183,9 @@ fn is_req(value: &str) -> bool {
     let Some(first) = value.chars().next() else {
         return false;
     };
-    "<>=^~".contains(first) || value.contains('*') || value.contains(',')
+    "<>=^~".contains(first)
+        || value.contains('*')
+        || value.contains(',')
+        || value.contains('x')
+        || value.contains('X')
 }

tests/testsuite/rust_version.rs

@@ -125,7 +125,6 @@ fn rust_version_bad_pre_release() {
 }
 
 #[cargo_test]
-#[should_panic]
 fn rust_version_x_wildcard() {
     project()
         .file(
@@ -150,16 +149,15 @@ fn rust_version_x_wildcard() {
 [ERROR] unexpected version requirement, expected a version like \"1.32\"
  --> Cargo.toml:7:28
   |
-7 |             rust-version = \"^1.43\"
-  |                            ^^^^^^^
+7 |             rust-version = \"x\"
+  |                            ^^^
   |
 ",
         )
         .run();
 }
 
 #[cargo_test]
-#[should_panic]
 fn rust_version_minor_x_wildcard() {
     project()
         .file(
@@ -193,7 +191,6 @@ fn rust_version_minor_x_wildcard() {
 }
 
 #[cargo_test]
-#[should_panic]
 fn rust_version_patch_x_wildcard() {
     project()
         .file(