Excavator: Update open-source publishing plugins (#364)

Author: svc-excavator-bot

build.gradle

@@ -5,8 +5,8 @@ buildscript {
     }
 
     dependencies {
-        classpath 'com.palantir.gradle.externalpublish:gradle-external-publish-plugin:1.7.0'
-        classpath 'com.gradle.publish:plugin-publish-plugin:0.19.0'
+        classpath 'com.palantir.gradle.externalpublish:gradle-external-publish-plugin:1.9.0'
+        classpath 'com.gradle.publish:plugin-publish-plugin:0.20.0'
         classpath 'com.palantir.baseline:gradle-baseline-java:4.59.0'
         classpath 'com.palantir.gradle.consistentversions:gradle-consistent-versions:2.5.0'
         classpath 'com.palantir.javaformat:gradle-palantir-java-format:2.15.0'