GEODE-10352: Update Ruby version in Geode doc preview tool (#7753)

The script to preview the documentation of Geode (./preview-user-guide.sh)
is not working anymore.

The following error appears while running it (unless you have a previously downloaded
geodedocs/temp docker image in your local docker repo):

ERROR: Error installing elasticsearch:
The last version of faraday (>= 0) to support your Ruby & RubyGems was 1.10.0. Try installing it with `gem install faraday -v 1.10.0` and then running the current command again
faraday requires Ruby version >= 2.6. The current ruby version is 2.5.9.229.

That error prevents the preview script to work.

It is needed to update the docker image used to preview the documentation to use a Ruby version >= 2.6.
The versions of other gems also need to be updated after the
change of the Ruby version.

dev-tools/docker/docs/Dockerfile

@@ -14,7 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM ruby:2.5
+FROM ruby:2.6.8
 
 LABEL Vendor="Apache Geode"
 LABEL version=unstable
@@ -25,16 +25,16 @@ RUN curl -fsSL https://deb.nodesource.com/setup_16.x | bash - ; \
     apt-get install -y nodejs
 RUN gem install bundler:1.17.3 \
     rake multi_json:1.13.1 \
-    elasticsearch:2.0.2 \
+    elasticsearch:7.5.0 \
     multipart-post:2.0.0 \
-    faraday:0.15.4 \
+    faraday:0.17.4 \
     libv8:3.16.14.15 \
-    mini_portile2:2.5.0 \
-    racc:1.5.2 \
-    nokogiri:1.11.2 \
-    mimemagic:0.3.9 \
-    puma:4.3.8 \
-    rack:2.1.4 \
+    mini_portile2:2.8.0 \
+    racc:1.6.0 \
+    nokogiri:1.13.3 \
+    mimemagic:0.3.10 \
+    puma:5.6.2 \
+    rack:2.2.3 \
     smtpapi:0.1.0 \
     sendgrid-ruby:1.1.6 \
     therubyracer:0.12.2