ci: replace Ubuntu2004 by Ubuntu2204

appveyor.yml

 # Setup different operating systems
 image:
   - Visual Studio 2019
-  - Ubuntu2004
+  - Ubuntu2204
   - macOS
 
 # Target x86_64
@@ -35,8 +35,7 @@ for:
   - # Linux setup
     matrix:
       only:
-        - image: Ubuntu2004
-        - image: Ubuntu1804
+        - image: Ubuntu2204
     install:
       - sudo apt-get update
       - sudo apt-get install -y build-essential cmake bison flex
@@ -55,4 +54,4 @@ before_build:
 
 # Build program
 build_script:
-  - cmake --build . --target install
\ No newline at end of file
+  - cmake --build . --target install