Bump plantuml from 1.2021.12 to 1.2021.15

Bumps plantuml from 1.2021.12 to 1.2021.15.

---
updated-dependencies:
- dependency-name: net.sourceforge.plantuml:plantuml
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

server/pom.xml

@@ -17,7 +17,7 @@
     <junit.jupiter.version>5.8.1</junit.jupiter.version>
     <assertj-core.version>3.21.0</assertj-core.version>
     <mockito-core.version>3.12.4</mockito-core.version>
-    <plantuml.version>1.2021.12</plantuml.version>
+    <plantuml.version>1.2021.15</plantuml.version>
     <ditaa-mini.version>0.14</ditaa-mini.version>
     <slf4j-api.version>1.7.32</slf4j-api.version>
     <maven.compiler.source>1.8</maven.compiler.source>