Downgrade package-not-found diagnostic to warning

error is reserved for when the build fails.

go/ql/integration-tests/all-platforms/go/diagnostics/package-not-found-with-go-mod/diagnostics.expected

@@ -1,6 +1,6 @@
 {
   "markdownMessage": "110 packages could not be found.\n\n`github.com/nosuchorg/nosuchrepo000`, `github.com/nosuchorg/nosuchrepo001`, `github.com/nosuchorg/nosuchrepo002`, `github.com/nosuchorg/nosuchrepo003`, `github.com/nosuchorg/nosuchrepo004` and 105 more.\n\nCheck that the paths are correct and make sure any private packages can be accessed. If any of the packages are present in the repository then you may need a [custom build command](https://docs.github.com/en/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-the-codeql-workflow-for-compiled-languages).",
-  "severity": "error",
+  "severity": "warning",
   "source": {
     "extractorName": "go",
     "id": "go/autobuilder/package-not-found",

go/ql/integration-tests/all-platforms/go/diagnostics/package-not-found-without-go-mod/diagnostics.expected

@@ -1,6 +1,6 @@
 {
   "markdownMessage": "1 package could not be found.\n\n`github.com/linode/linode-docs-theme`.\n\nCheck that the paths are correct and make sure any private packages can be accessed. If any of the packages are present in the repository then you may need a [custom build command](https://docs.github.com/en/code-security/code-scanning/automatically-scanning-your-code-for-vulnerabilities-and-errors/configuring-the-codeql-workflow-for-compiled-languages).",
-  "severity": "error",
+  "severity": "warning",
   "source": {
     "extractorName": "go",
     "id": "go/autobuilder/package-not-found",