Upgrade xtext dependency to 2.26.0. (89f5657)

https://www.eclipse.org/Xtext/releasenotes.html#/releasenotes/2022/02/28/version-2-26-0

Author: blackwinter

build.gradle

@@ -40,7 +40,7 @@ subprojects {
       'mockito':        '2.27.0',
       'requirejs':      '2.3.6',
       'slf4j':          '1.7.21',
-      'xtext':          '2.26.0.M2',
+      'xtext':          '2.26.0',
       'guava':          '29.0-jre'
     ]
   }