Remove fedora 26, add 28 (#30683)

* Remove fedora 26, add 28

Closes #30579

* Update testing docs with new fedora 28 image

TESTING.asciidoc

@@ -379,7 +379,7 @@ You can choose which boxes to test by setting the `-Pvagrant.boxes` project prop
 the valid options for this property are:
 
 * `sample` - The default, only chooses ubuntu-1404 and centos-7
-* List of box names, comma separated (e.g. `oel-7,fedora-26`) - Chooses exactly the boxes listed.
+* List of box names, comma separated (e.g. `oel-7,fedora-28`) - Chooses exactly the boxes listed.
 * `linux-all` - All linux boxes.
 * `windows-all` - All Windows boxes. If there are any Windows boxes which do not
 have images available when this value is provided, the build will fail.
@@ -406,8 +406,8 @@ These are the linux flavors supported, all of which we provide images for
 * debian-9 aka stretch, the current debian stable distribution
 * centos-6
 * centos-7
-* fedora-26
 * fedora-27
+* fedora-28
 * oel-6 aka Oracle Enterprise Linux 6
 * oel-7 aka Oracle Enterprise Linux 7
 * sles-12

Vagrantfile

@@ -97,15 +97,15 @@ Vagrant.configure(2) do |config|
       rpm_common config, box
     end
   end
-  'fedora-26'.tap do |box|
+  'fedora-27'.tap do |box|
     config.vm.define box, define_opts do |config|
-      config.vm.box = 'elastic/fedora-26-x86_64'
+      config.vm.box = 'elastic/fedora-27-x86_64'
       dnf_common config, box
     end
   end
-  'fedora-27'.tap do |box|
+  'fedora-28'.tap do |box|
     config.vm.define box, define_opts do |config|
-      config.vm.box = 'elastic/fedora-27-x86_64'
+      config.vm.box = 'elastic/fedora-28-x86_64'
       dnf_common config, box
     end
   end

buildSrc/src/main/groovy/org/elasticsearch/gradle/vagrant/VagrantTestPlugin.groovy

@@ -23,8 +23,8 @@ class VagrantTestPlugin implements Plugin<Project> {
             'centos-7',
             'debian-8',
             'debian-9',
-            'fedora-26',
             'fedora-27',
+            'fedora-28',
             'oel-6',
             'oel-7',
             'opensuse-42',