Upgrade: [dependabot] - bump fhir.version from 7.6.1 to 8.4.0

Bumps `fhir.version` from 7.6.1 to 8.4.0.

Updates `ca.uhn.hapi.fhir:hapi-fhir-validation-resources-r4` from 7.6.1 to 8.4.0

Updates `ca.uhn.hapi.fhir:hapi-fhir-structures-r4` from 7.6.1 to 8.4.0

Updates `ca.uhn.hapi.fhir:hapi-fhir-validation` from 7.6.1 to 8.4.0

Updates `ca.uhn.hapi.fhir:hapi-fhir-caching-caffeine` from 7.6.1 to 8.4.0

Updates `ca.uhn.hapi.fhir:hapi-fhir-base` from 7.6.1 to 8.4.0

---
updated-dependencies:
- dependency-name: ca.uhn.hapi.fhir:hapi-fhir-validation-resources-r4
  dependency-version: 8.4.0
  dependency-type: direct:production
  update-type: version-update:semver-major
- dependency-name: ca.uhn.hapi.fhir:hapi-fhir-structures-r4
  dependency-version: 8.4.0
  dependency-type: direct:production
  update-type: version-update:semver-major
- dependency-name: ca.uhn.hapi.fhir:hapi-fhir-validation
  dependency-version: 8.4.0
  dependency-type: direct:production
  update-type: version-update:semver-major
- dependency-name: ca.uhn.hapi.fhir:hapi-fhir-caching-caffeine
  dependency-version: 8.4.0
  dependency-type: direct:production
  update-type: version-update:semver-major
- dependency-name: ca.uhn.hapi.fhir:hapi-fhir-base
  dependency-version: 8.4.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

pom.xml

@@ -18,7 +18,7 @@
         <maven.compiler.source>21</maven.compiler.source>
         <maven.compiler.target>21</maven.compiler.target>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-        <fhir.version>7.6.1</fhir.version>
+        <fhir.version>8.4.0</fhir.version>
         <log4j.version>2.24.3</log4j.version>
         <fasterxml.version>2.19.2</fasterxml.version>
         <sonar.organization>nhsdigital</sonar.organization>