Container tests fail because MockServer client and server versions do not match

[LouisLou2]

The container tests currently use different MockServer client and server versions.

#9622 updated mockserver-client-java-no-dependencies from 7.4.0 to 7.5.0, but
BasicMockServerTest.MOCKSERVER_IMAGE still uses the
mockserver/mockserver:mockserver-7.4.0 image.

MockServer rejects requests when the client and server major or minor versions
do not match. As a result, all container tests fail while registering
expectations, before making an OkHttp request.

Reproduction

From the current main branch:

./gradlew :container-tests:test -PcontainerTests=true

This produces failures such as:

org.mockserver.client.ClientException:
Client version "7.5.0" major and minor versions do not match
server version "7.4.0"

The Loom configuration reproduces the same problem:

./gradlew :container-tests:test \
  -PcontainerTests=true \
  -Pokhttp.platform=loom \
  -Ptest.java.version=24

All 12 container tests fail in the Loom configuration. The same failure can
also be seen in the Loom job for #9622:

https://github.com/lysine-dev/okhttp/actions/runs/30490484586/job/90706867182

Expected behavior

The MockServer client and container image use compatible versions, and the
container tests pass.

Proposed fix

Update the container image to match the client version:

-.withTag("mockserver-7.4.0")
+.withTag("mockserver-7.5.0")

I verified this change with both commands above. All 12 container tests pass
with the Loom configuration.