Upgrade openapi plugin version from 7.7 to 7.11.

In version 7.7 generating documentation via the openApi gradle task results in the following misleading warning:

"More than one inline schema specified in allOf:. Only the first one is recognized. All others are ignored."

See comments on this [PR](OpenAPITools/openapi-generator#19646).

In later versions of the plugin - and rather than fixing the issue - the severity of the warning has been reduced from [WARN to DEBUG](https://github.com/OpenAPITools/openapi-generator/pull/19646/files).

Also set removeEnumValuePrefix = true since this is a new additional property added to more recent versions which removes the prefix for enum definitions.  Previously in version 7.7 prefixes were removed automatically and the property didn't exist.  Since it [defaults to false](OpenAPITools/openapi-generator#20452), we need to set it to true.  Unfortunately this property isn't documented in the [README](OpenAPITools/openapi-generator#20452).

build.gradle.kts

@@ -3,7 +3,7 @@ import org.apache.commons.io.FileUtils
 plugins {
   id("uk.gov.justice.hmpps.gradle-spring-boot") version "6.1.0"
   kotlin("plugin.spring") version "2.0.20"
-  id("org.openapi.generator") version "7.7.0"
+  id("org.openapi.generator") version "7.11.0"
   id("org.jetbrains.kotlin.plugin.jpa") version "1.9.22"
   id("io.gatling.gradle") version "3.13.1"
   id("io.gitlab.arturbosch.detekt") version "1.23.7"
@@ -239,6 +239,7 @@ openApiGenerate {
   typeMappings.put("DateTime", "Instant")
   importMappings.put("Instant", "java.time.Instant")
   templateDir.set("$rootDir/openapi")
+  additionalProperties.put("removeEnumValuePrefix", "true")
 }
 
 registerAdditionalOpenApiGenerateTask(