Bump log4j-api version to 2.15.0

The main log4j version cannot be increased yet due to some incompatibilities with the Qortal code.

pom.xml

@@ -23,6 +23,7 @@
 		<jersey.version>2.29.1</jersey.version>
 		<jetty.version>9.4.29.v20200521</jetty.version>
 		<log4j.version>2.12.1</log4j.version>
+		<log4j-api.version>2.15.0</log4j-api.version>
 		<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 		<slf4j.version>1.7.12</slf4j.version>
 		<swagger-api.version>2.0.9</swagger-api.version>
@@ -518,7 +519,7 @@
 		<dependency>
 			<groupId>org.apache.logging.log4j</groupId>
 			<artifactId>log4j-api</artifactId>
-			<version>${log4j.version}</version>
+			<version>${log4j-api.version}</version>
 		</dependency>
 		<!-- redirect slf4j to log4j2 -->
 		<dependency>