Mark Log4j API dependency as non-optional

The Log4j dependency is separated into two artifacts, the API and the
core implementation. This is to enable replacing Log4j on the backend
through the SLF4J bridge with another logging implementation. For this
reason, the dependencies are marked as optional. This causes confusion
amongst users as to use the bridge, the API should be non-optional since
it is needed for the bridge to function correctly. While they could pull
it into their application directly, it would be clearer if we simply
marked this depdendency as non-optional. Note that this does not mean
that users have to use Log4j for logging in their application, so we are
not marking core as required, it only clarifies what they need to be
able to plug in a different logging implementation.

Relates #25136

core/build.gradle

@@ -81,7 +81,7 @@ dependencies {
   compile "com.vividsolutions:jts:${versions.jts}", optional
 
   // logging
-  compile "org.apache.logging.log4j:log4j-api:${versions.log4j}", optional
+  compile "org.apache.logging.log4j:log4j-api:${versions.log4j}"
   compile "org.apache.logging.log4j:log4j-core:${versions.log4j}", optional
   // to bridge dependencies that are still on Log4j 1 to Log4j 2
   compile "org.apache.logging.log4j:log4j-1.2-api:${versions.log4j}", optional

docs/java-api/index.asciidoc

@@ -41,11 +41,6 @@ You need to also include Log4j 2 dependencies:
 
 ["source","xml",subs="attributes"]
 --------------------------------------------------
-<dependency>
-    <groupId>org.apache.logging.log4j</groupId>
-    <artifactId>log4j-api</artifactId>
-    <version>2.8.2</version>
-</dependency>
 <dependency>
     <groupId>org.apache.logging.log4j</groupId>
     <artifactId>log4j-core</artifactId>