Build(deps): bump springCloudContractVersion from 3.1.7 to 3.1.8

Bumps `springCloudContractVersion` from 3.1.7 to 3.1.8.

Updates `org.springframework.cloud:spring-cloud-contract-gradle-plugin` from 3.1.7 to 3.1.8

Updates `org.springframework.cloud:spring-cloud-contract-dependencies` from 3.1.7 to 3.1.8
- [Release notes](https://github.com/spring-cloud/spring-cloud-build/releases)
- [Commits](spring-cloud/spring-cloud-build@v3.1.7...v3.1.8)

---
updated-dependencies:
- dependency-name: org.springframework.cloud:spring-cloud-contract-gradle-plugin
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.springframework.cloud:spring-cloud-contract-dependencies
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

applications/credhub-api/build.gradle

@@ -1,6 +1,6 @@
 buildscript {
     ext {
-        springCloudContractVersion = '3.1.7'
+        springCloudContractVersion = '3.1.8'
     }
     repositories {
         mavenCentral()