Use ubuntu-18.04 instead of ubuntu-16.04

Author: taiki-e

ci/azure-clippy.yml

@@ -3,7 +3,7 @@ jobs:
   displayName: Clippy
 
   pool:
-    vmImage: ubuntu-16.04
+    vmImage: ubuntu-18.04
 
   steps:
     - template: azure-install-rust.yml

ci/azure-cross-compile.yml

@@ -1,5 +1,5 @@
 parameters:
-  vmImage: ubuntu-16.04
+  vmImage: ubuntu-18.04
 
 jobs:
   - job: ${{ parameters.name }}
@@ -15,23 +15,23 @@ jobs:
           target: aarch64-apple-ios
 
         Android_ARM:
-          vmImage: ubuntu-16.04
+          vmImage: ubuntu-18.04
           target: arm-linux-androideabi
 
         Android_ARM64:
-          vmImage: ubuntu-16.04
+          vmImage: ubuntu-18.04
           target: aarch64-linux-android
 
         Android_32:
-          vmImage: ubuntu-16.04
+          vmImage: ubuntu-18.04
           target: i686-unknown-linux-gnu
 
         NetBSD:
-          vmImage: ubuntu-16.04
+          vmImage: ubuntu-18.04
           target: x86_64-unknown-netbsd
 
         Solaris:
-          vmImage: ubuntu-16.04
+          vmImage: ubuntu-18.04
           target: x86_64-pc-solaris
 
     pool:

ci/azure-minimal-versions.yml

@@ -7,7 +7,7 @@ jobs:
     strategy:
       matrix:
         Linux:
-          vmImage: ubuntu-16.04
+          vmImage: ubuntu-18.04
         Windows:
           vmImage: vs2017-win2016
     pool:

ci/azure-rustfmt.yml

@@ -3,7 +3,7 @@ jobs:
   - job: ${{ parameters.name }}
     displayName: Check rustfmt
     pool:
-      vmImage: ubuntu-16.04
+      vmImage: ubuntu-18.04
     steps:
       - template: azure-install-rust.yml
         parameters:

ci/azure-test-stable.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         Linux:
-          vmImage: ubuntu-16.04
+          vmImage: ubuntu-18.04
 
         ${{ if parameters.cross }}:
           MacOS: