Jenkinsfile: Update deprecated 'stage' syntax

Jenkinsfile

@@ -1,10 +1,12 @@
+#!/usr/bin/env groovy
 properties([[$class: 'BuildDiscarderProperty', strategy: [$class: 'LogRotator', artifactDaysToKeepStr: '30', artifactNumToKeepStr: '20']]])
 
 node('master') {
-    stage 'Pull changes'
+    stage('Pull changes') {
         checkout scm
+    }
 
-    stage 'Build Windows'
+    stage('Build Windows') {
         sh 'mkdir -p build/windows'
         dir('build/windows') {
             sh '''
@@ -19,8 +21,9 @@ node('master') {
         }
         sh 'rm -f windows-debug.zip'
         zip zipFile: 'windows-debug.zip', archive: true, dir: 'build/windows/install'
+    }
 
-    stage 'Build Linux'
+    stage('Build Linux') {
         sh 'mkdir -p build/linux'
         dir('build/linux') {
             sh '''
@@ -36,17 +39,19 @@ node('master') {
         }
         sh 'rm -f linux-debug.zip'
         zip zipFile: 'linux-debug.zip', archive: true, dir: 'build/linux/install'
+    }
 
-    stage 'Doxygen'
+    stage('Doxygen') {
         dir('build/linux') {
             sh 'make doc'
         }
         publishHTML target: [$class: 'HtmlPublisherTarget', reportName: 'Doxygen', reportDir: 'build/linux/doc/html', reportFiles: 'index.html']
+    }
 
-    stage 'Run tests'
+    stage('Run tests') {
         dir('build/linux') {
             sh './colobot_ut --gtest_output=xml:gtestresults.xml || true'
         }
         step([$class: 'XUnitBuilder', testTimeMargin: '3000', thresholdMode: 1, thresholds: [[$class: 'FailedThreshold', failureNewThreshold: '', failureThreshold: '', unstableNewThreshold: '', unstableThreshold: '0'], [$class: 'SkippedThreshold', failureNewThreshold: '', failureThreshold: '', unstableNewThreshold: '', unstableThreshold: '']], tools: [[$class: 'GoogleTestType', deleteOutputFiles: true, failIfNotNew: true, pattern: 'build/linux/gtestresults.xml', skipNoTestFiles: false, stopProcessingIfError: true]]])
+    }
 }
-

data

@@ -1 +1 @@
-Subproject commit 41c03fda84e6eab6cd105fffd77a26cde4c1fe12
+Subproject commit b325c53316b0b4ac7fe69597bff69499bee0ef6a