Version mismatch in springdoc-openapi-tests: sub-modules still reference 2.8.17-SNAPSHOT after version bump to 2.8.18

## Description

When cloning the repository and running `mvn clean install -DskipTests` from 
the root, the build fails immediately with:

[FATAL] Non-resolvable parent POM for org.springdoc:springdoc-openapi-tests:2.8.17-SNAPSHOT
Could not find artifact org.springdoc:springdoc-openapi:pom:2.8.17-SNAPSHOT

## Root Cause

The root `pom.xml` was bumped to `2.8.18-SNAPSHOT`, but the following modules 
were not updated and still reference `2.8.17-SNAPSHOT` as their parent version:

- `springdoc-openapi-tests/pom.xml`
- `springdoc-openapi-tests/springdoc-openapi-security-tests/pom.xml`
- `springdoc-openapi-tests/springdoc-openapi-groovy-tests/pom.xml`
- `springdoc-openapi-tests/springdoc-openapi-javadoc-tests/pom.xml`
- `springdoc-openapi-tests/springdoc-openapi-function-webmvc-tests/pom.xml`
- `springdoc-openapi-tests/springdoc-openapi-function-webflux-tests/pom.xml`
- `springdoc-openapi-tests/springdoc-openapi-actuator-webflux-tests/pom.xml`
- `springdoc-openapi-tests/springdoc-openapi-actuator-webmvc-tests/pom.xml`
- `springdoc-openapi-tests/springdoc-openapi-kotlin-webflux-tests/pom.xml`
- `springdoc-openapi-tests/springdoc-openapi-kotlin-webmvc-tests/pom.xml`
- `springdoc-openapi-tests/springdoc-openapi-hateoas-tests/pom.xml`
- `springdoc-openapi-tests/springdoc-openapi-data-rest-tests/pom.xml`

## Steps to Reproduce

```bash
git clone https://github.com/springdoc/springdoc-openapi.git
cd springdoc-openapi
mvn clean install -DskipTests
```

## Expected Result
Build succeeds.

## Actual Result
Build fails — 11 modules cannot resolve their parent POM.

## Fix
Update all affected `pom.xml` files to reference `2.8.18-SNAPSHOT`.

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