Bump testContainersVersion from 1.20.6 to 1.21.0 in /Plan (#4084)

Bumps `testContainersVersion` from 1.20.6 to 1.21.0.

Updates `org.testcontainers:testcontainers` from 1.20.6 to 1.21.0
- [Release notes](https://github.com/testcontainers/testcontainers-java/releases)
- [Changelog](https://github.com/testcontainers/testcontainers-java/blob/main/CHANGELOG.md)
- [Commits](testcontainers/testcontainers-java@1.20.6...1.21.0)

Updates `org.testcontainers:junit-jupiter` from 1.20.6 to 1.21.0
- [Release notes](https://github.com/testcontainers/testcontainers-java/releases)
- [Changelog](https://github.com/testcontainers/testcontainers-java/blob/main/CHANGELOG.md)
- [Commits](testcontainers/testcontainers-java@1.20.6...1.21.0)

Updates `org.testcontainers:nginx` from 1.20.6 to 1.21.0
- [Release notes](https://github.com/testcontainers/testcontainers-java/releases)
- [Changelog](https://github.com/testcontainers/testcontainers-java/blob/main/CHANGELOG.md)
- [Commits](testcontainers/testcontainers-java@1.20.6...1.21.0)

---
updated-dependencies:
- dependency-name: org.testcontainers:testcontainers
  dependency-version: 1.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: org.testcontainers:junit-jupiter
  dependency-version: 1.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
- dependency-name: org.testcontainers:nginx
  dependency-version: 1.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Plan/build.gradle

@@ -93,7 +93,7 @@ subprojects {
         junitVersion = "5.12.2"
         mockitoVersion = "5.18.0"
         seleniumVersion = "4.32.0"
-        testContainersVersion = "1.20.6"
+        testContainersVersion = "1.21.0"
         awaitilityVersion = "4.3.0"
         swaggerVersion = "2.2.32"
         guavaVersion = "33.4.8-jre"