Upgrade Maven plugin dependencies

- upgrade asciidoctor-maven-plugin to 2.2.5
- upgrade exec-maven-plugin to 3.1.1
- upgrade flatten-maven-plugin to 1.6.0
- upgrade maven-compiler-plugin to 3.12.1
- upgrade maven-javadoc-plugin to 3.6.3
- upgrade maven-surefire-plugin to 3.2.5

pom.xml

@@ -111,29 +111,29 @@
 		<source.branch>main</source.branch>
 
 		<!-- plugin dependency versions (alphabetically) -->
-		<asciidoctor-maven-plugin.version>2.2.4</asciidoctor-maven-plugin.version>
+		<asciidoctor-maven-plugin.version>2.2.5</asciidoctor-maven-plugin.version>
 		<build-helper-maven-plugin.version>3.5.0</build-helper-maven-plugin.version>
 		<doxia-module-markdown.version>1.0</doxia-module-markdown.version>
-		<exec-maven-plugin.version>3.1.0</exec-maven-plugin.version>
-		<flatten-maven-plugin.version>1.5.0</flatten-maven-plugin.version>
+		<exec-maven-plugin.version>3.1.1</exec-maven-plugin.version>
+		<flatten-maven-plugin.version>1.6.0</flatten-maven-plugin.version>
 		<github.site-maven-plugin.version>0.12</github.site-maven-plugin.version>
 		<maven-compiler-plugin.version>3.12.1</maven-compiler-plugin.version>
 		<dependency-check-maven.version>9.0.9</dependency-check-maven.version>
 		<maven-antrun-plugin.version>3.1.0</maven-antrun-plugin.version>
 		<maven-checkstyle-plugin.version>3.3.1</maven-checkstyle-plugin.version>
-		<maven-compiler-plugin.version>3.8.1</maven-compiler-plugin.version>
+		<maven-compiler-plugin.version>3.12.1</maven-compiler-plugin.version>
 		<maven-deploy-plugin.version>3.1.1</maven-deploy-plugin.version>
 		<maven-enforcer-plugin.version>3.4.1</maven-enforcer-plugin.version>
 		<maven-gpg-plugin.version>3.1.0</maven-gpg-plugin.version>
 		<maven-install-plugin.version>3.1.1</maven-install-plugin.version>
 		<maven-jacoco-plugin.version>0.8.11</maven-jacoco-plugin.version>
 		<maven-jar-plugin.version>3.3.0</maven-jar-plugin.version>
-		<maven-javadoc-plugin.version>3.5.0</maven-javadoc-plugin.version>
+		<maven-javadoc-plugin.version>3.6.3</maven-javadoc-plugin.version>
 		<maven-release-plugin.version>3.0.1</maven-release-plugin.version>
 		<maven-resources-plugin.version>3.3.1</maven-resources-plugin.version>
 		<maven-site-plugin.version>3.12.1</maven-site-plugin.version>
 		<maven-source-plugin.version>3.3.0</maven-source-plugin.version>
-		<maven-surefire-plugin.version>3.2.3</maven-surefire-plugin.version>
+		<maven-surefire-plugin.version>3.2.5</maven-surefire-plugin.version>
 		<nexus-staging-maven-plugin.version>1.6.13</nexus-staging-maven-plugin.version>
 		<spring-boot-maven-plugin.version>${spring-boot.version}</spring-boot-maven-plugin.version>