read to buffer not string and use non-legacy method to base64

src/create-pull-request.ts

@@ -327,7 +327,7 @@ export async function createPullRequest(inputs: Inputs): Promise<void> {
           core.debug(`Reading contents of file: '${file}'`)
           fileChanges.additions!.push({
             path: file,
-            contents: btoa(fs.readFileSync(file, 'utf8'))
+            contents: fs.readFileSync(file).toString('base64')
           })
         }