Excavator: Upgrades Baseline to the latest version (#326)

###### _excavator_ is a bot for automating changes across repositories.

Changes produced by the roomba/latest-baseline-oss check.

# Release Notes
## 4.19.0
| Type | Description | Link |
| ---- | ----------- | ---- |
| Fix | Re-add null check for exclude rule | palantir/gradle-baseline#1883 |


## 4.20.0
| Type | Description | Link |
| ---- | ----------- | ---- |
| Fix | Fix missing setProperty error when using Gradle 6 | palantir/gradle-baseline#1884 |



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build.gradle

@@ -7,7 +7,7 @@ buildscript {
     dependencies {
         classpath 'com.palantir.gradle.externalpublish:gradle-external-publish-plugin:1.2.1'
         classpath 'com.gradle.publish:plugin-publish-plugin:0.15.0'
-        classpath 'com.palantir.baseline:gradle-baseline-java:4.18.0'
+        classpath 'com.palantir.baseline:gradle-baseline-java:4.20.0'
         classpath 'com.palantir.gradle.conjure:gradle-conjure:5.10.0'
         classpath 'com.palantir.gradle.consistentversions:gradle-consistent-versions:2.0.0'
         classpath 'com.palantir.gradle.gitversion:gradle-git-version:0.12.3'