Include fontconfig in Debian Linux install (jenkins-infra#6721)

Include fontconfig package in Debian install

Avoid runtime warning messages from Jenkins core when Java attempts to
access font definitions.

Increase consistency with the container images that install either
`fontconfig`  or `libfontconfig1` in order to avoid warning messages
from Jenkins core when Java attempts to access font definitions.

Increase consistency with the configuration tested in the Jenkins
packaging repository.  The packaging repository tests with `fontconfig`.

content/doc/book/installing/linux.adoc

@@ -97,11 +97,11 @@ Update the Debian apt repositories, install OpenJDK 17, and check the installati
 [source,bash]
 ----
 sudo apt update
-sudo apt install openjdk-17-jre
+sudo apt install fontconfig openjdk-17-jre
 java -version
-openjdk version "17.0.7" 2023-04-18
-OpenJDK Runtime Environment (build 17.0.7+7-Debian-1deb11u1)
-OpenJDK 64-Bit Server VM (build 17.0.7+7-Debian-1deb11u1, mixed mode, sharing)
+openjdk version "17.0.8" 2023-07-18
+OpenJDK Runtime Environment (build 17.0.8+7-Debian-1deb12u1)
+OpenJDK 64-Bit Server VM (build 17.0.8+7-Debian-1deb12u1, mixed mode, sharing)
 ----
 
 [NOTE]