TravisCI: switch to bionic for build environment

Keep using xenial for legacy PHP versions (5.6 and 7.0) as they are not
supported on bionic.

Fixes #27829

.travis.yml

@@ -4,7 +4,7 @@
 
 language: php
 os: linux
-dist: xenial
+dist: bionic
 
 # -----------------------------------------------------------------------------
 # Environment setup and test scripts execution
@@ -32,8 +32,6 @@ script:
 #
 
 php:
-  - 5.6
-  - 7.0
   - 7.1
   - 7.2
   - 7.3
@@ -46,6 +44,20 @@ env:
 
 jobs:
   include:
+    # Use 'xenial' build environment for legacy PHP versions
+    - php: 5.6
+      dist: xenial
+      env: DB=mysql
+    - php: 5.6
+      dist: xenial
+      env: DB=pgsql
+    - php: 7.0
+      dist: xenial
+      env: DB=mysql
+    - php: 7.0
+      dist: xenial
+      env: DB=pgsql
+
     # Add a specific build for Documentation
     - name: Documentation
       env: DOCBOOK=1