Bump com.io7m.xstructural.version from 1.8.0 to 1.8.1

Bumps `com.io7m.xstructural.version` from 1.8.0 to 1.8.1.

Updates `com.io7m.xstructural:com.io7m.xstructural.vanilla` from 1.8.0 to 1.8.1
- [Changelog](https://github.com/io7m-com/xstructural/blob/develop/README-CHANGES.xml)
- [Commits](io7m-com/xstructural@com.io7m.xstructural-1.8.0...com.io7m.xstructural-1.8.1)

Updates `com.io7m.xstructural:com.io7m.xstructural.maven_plugin` from 1.8.0 to 1.8.1
- [Changelog](https://github.com/io7m-com/xstructural/blob/develop/README-CHANGES.xml)
- [Commits](io7m-com/xstructural@com.io7m.xstructural-1.8.0...com.io7m.xstructural-1.8.1)

---
updated-dependencies:
- dependency-name: com.io7m.xstructural:com.io7m.xstructural.vanilla
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: com.io7m.xstructural:com.io7m.xstructural.maven_plugin
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

pom.xml

@@ -64,7 +64,7 @@
     <com.io7m.jbssio.version>2.0.0</com.io7m.jbssio.version>
     <com.io7m.jsx.version>3.0.1</com.io7m.jsx.version>
     <com.io7m.quarrel.version>1.6.0</com.io7m.quarrel.version>
-    <com.io7m.xstructural.version>1.8.0</com.io7m.xstructural.version>
+    <com.io7m.xstructural.version>1.8.1</com.io7m.xstructural.version>
     <io7m.maven-api.version>3.9.6</io7m.maven-api.version>
 
     <!-- Configuration. -->