update workflows to use gcc 11/13/15

instead of gcc 10/12, because it's not 2023 anymore

Author: ab9rf

.github/workflows/build-linux.yml

@@ -66,7 +66,7 @@ on:
         default: false
       gcc-ver:
         type: string
-        default: "10"
+        default: "11"
 
 jobs:
   build-linux64:

.github/workflows/test.yml

@@ -59,9 +59,11 @@ jobs:
       fail-fast: false
       matrix:
         include:
-        - gcc: 10
+        - gcc: 11 # baseline compatibility with ubuntu LTS 22.04
           plugins: "default"
-        - gcc: 12
+        - gcc: 13 # future compatibility with ubuntu LTS 24.04
+          plugins: "all"
+        - gcc: 15 # cutting-edge
           plugins: "all"
 
   test-windows: