Change v3 deprecation message to warning.

CHANGELOG.md

@@ -4,7 +4,7 @@ See the [releases page](https://github.com/github/codeql-action/releases) for th
 
 ## [UNRELEASED]
 
-No user facing changes.
+- Downgraded the severity of the "v3 deprecation" notice from "error" to "warning", since the deprecation notice is meant to _warn_ users in advance.
 
 ## 4.31.3 - 13 Nov 2025
 

src/util.test.ts

@@ -476,7 +476,7 @@ for (const [
     githubVersion,
   )}`;
   test(`checkActionVersion ${reportErrorDescription} for ${versionsDescription}`, async (t) => {
-    const warningSpy = sinon.spy(core, "error");
+    const warningSpy = sinon.spy(core, "warning");
     const versionStub = sinon
       .stub(api, "getGitHubVersion")
       .resolves(githubVersion);

src/util.ts

@@ -1141,7 +1141,7 @@ export function checkActionVersion(
           ">=3.20",
         ))
     ) {
-      core.error(
+      core.warning(
         "CodeQL Action v3 will be deprecated in December 2026. " +
           "Please update all occurrences of the CodeQL Action in your workflow files to v4. " +
           "For more information, see " +