chore(kokoro): fix java8-osx lock acquisition failure (#14812)

## Problem
- Parallel (`-T 1C`) Maven installs on slower macOS runners can wait
longer than the resolver named-lock timeout (30s by default before Maven
3.9.12) and fail with `Could not acquire lock(s)`.

## Solution
- Set `-Daether.syncContext.named.time=900` in `.mvn/maven.config`,
matching the Maven 3.9.12+ default, so every Maven invocation in the
repo gets the longer lock wait regardless of the Maven version installed
on the runner or how scripts set `MAVEN_OPTS`. Resolver locking stays
enabled.

Author: sakthivelmanii

.mvn/maven.config

@@ -1,2 +1,3 @@
 -Daether.remoteRepositoryFilter.prefixes=false
 -Daether.remoteRepositoryFilter.groupId=false
+-Daether.syncContext.named.time=900