Bump org.agrona:agrona from 1.6.0 to 1.21.1

Bumps [org.agrona:agrona](https://github.com/real-logic/agrona) from 1.6.0 to 1.21.1.
- [Release notes](https://github.com/real-logic/agrona/releases)
- [Commits](aeron-io/agrona@1.6.0...1.21.1)

---
updated-dependencies:
- dependency-name: org.agrona:agrona
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

pom.xml

@@ -45,7 +45,7 @@
     <checkstyle.config.location>io/aklivity/zilla/conf/checkstyle/configuration.xml</checkstyle.config.location>
     <checkstyle.suppressions.location>io/aklivity/zilla/conf/checkstyle/suppressions.xml</checkstyle.suppressions.location>
     <antlr4.version>4.13.0</antlr4.version>
-    <agrona.version>1.6.0</agrona.version>
+    <agrona.version>1.21.1</agrona.version>
     <slf4j.version>1.7.36</slf4j.version>
     <junit.version>5.10.1</junit.version>
     <byteman.version>4.0.22</byteman.version>