chore(deps): Enforce Maven version 3.6.3 or later

During the update of dependencies, Maven advised to do this. Version
3.6.3 of Maven is technically already "end of life" but, "currently,
plugins provide Maven API compatibility down to 3.6.3"
(https://maven.apache.org/docs/history.html 205-06-04) so we do this
too. Note that we do not currently have any requirement to use any
later version so this is totally fine.

Author: ibbem

pom.xml

@@ -20,6 +20,27 @@
 
     <build>
         <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-enforcer-plugin</artifactId>
+                <version>3.5.0</version>
+                <executions>
+                <execution>
+                    <id>enforce-maven</id>
+                    <goals>
+                        <goal>enforce</goal>
+                    </goals>
+                    <configuration>
+                    <rules>
+                        <requireMavenVersion>
+                        <version>3.6.3</version>
+                        </requireMavenVersion>
+                    </rules>
+                    </configuration>
+                </execution>
+                </executions>
+            </plugin>
+
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-javadoc-plugin</artifactId>