Adjust github ci runner images (#3040)

* Adjust github ci runner images

* fix name

Author: davisking

.github/workflows/build_cpp.yml

@@ -30,8 +30,8 @@ defaults:
     working-directory: dlib/test
 
 jobs:
-  ubuntu-latest-gcc-default-cmake-3-8-ffmpeg5:
-    runs-on: 'ubuntu-latest'
+  ubuntu-22-04-gcc-default-cmake-3-8-ffmpeg5:
+    runs-on: 'ubuntu-22.04'
     steps:
     - uses: actions/checkout@v2
 
@@ -195,8 +195,8 @@ jobs:
     - name: Build examples, etc
       run: cmake --build build --config Release --parallel 2
 
-  ubuntu-latest-ffmpeg7:
-    runs-on: 'ubuntu-latest'
+  ubuntu-22-04-ffmpeg7:
+    runs-on: 'ubuntu-22.04'
     steps:
     - uses: actions/checkout@v2
 

.github/workflows/build_matlab.yml

@@ -32,7 +32,7 @@ defaults:
 
 jobs:
   mex-wrapper:
-    runs-on: 'ubuntu-latest'
+    runs-on: 'ubuntu-22.04'
     steps:
     - uses: actions/checkout@v2
     - name: Setup MATLAB