build(deps): update dependency org.codehaus.mojo:animal-sniffer-annotations to v1.21

Author: renovate-bot

third-party-dependencies/pom.xml

@@ -60,7 +60,7 @@
  <gson.version>2.8.9</gson.version>
  <threeten.version>1.5.2</threeten.version>
  <javax.annotations.version>1.3.2</javax.annotations.version>
- <animal-sniffer.version>1.20</animal-sniffer.version>
+ <animal-sniffer.version>1.21</animal-sniffer.version>
  <opencensus.version>0.31.0</opencensus.version>
  <findbugs.version>3.0.2</findbugs.version>
  <errorprone.version>2.11.0</errorprone.version>