Downgrade flatten-maven-plugin to 1.7.3

Update the root Maven property to use `flatten-maven-plugin` 1.7.3 instead of 1.8.0, likely to keep the build aligned with a compatible/stable plugin version.

Author: mercyblitz

pom.xml

@@ -90,7 +90,7 @@
         <maven-gpg-plugin.version>3.2.8</maven-gpg-plugin.version>
         <sign-maven-plugin.version>1.1.0</sign-maven-plugin.version>
         <nexus-staging-maven-plugin.version>1.7.0</nexus-staging-maven-plugin.version>
-        <flatten-maven-plugin.version>1.8.0</flatten-maven-plugin.version>
+        <flatten-maven-plugin.version>1.7.3</flatten-maven-plugin.version>
         <asciidoctor-maven-plugin.version>3.2.0</asciidoctor-maven-plugin.version>
         <docbkx-maven-plugin.version>2.0.17</docbkx-maven-plugin.version>
         <build-helper-maven-plugin.version>3.6.1</build-helper-maven-plugin.version>