Upgrade errorprone-slf4j 0.1.18 -> 0.1.20 (#741)

See:
- https://github.com/KengoTODA/errorprone-slf4j/releases/tag/v0.1.19
- https://github.com/KengoTODA/errorprone-slf4j/releases/tag/v0.1.20
- KengoTODA/errorprone-slf4j@v0.1.18...v0.1.20

pom.xml

@@ -202,7 +202,7 @@
         <version.error-prone>${version.error-prone-orig}</version.error-prone>
         <version.error-prone-fork>v${version.error-prone-orig}-picnic-1</version.error-prone-fork>
         <version.error-prone-orig>2.21.1</version.error-prone-orig>
-        <version.error-prone-slf4j>0.1.18</version.error-prone-slf4j>
+        <version.error-prone-slf4j>0.1.20</version.error-prone-slf4j>
         <version.guava-beta-checker>1.0</version.guava-beta-checker>
         <version.jdk>11</version.jdk>
         <version.maven>3.8.7</version.maven>