[9.1] Allow xpack.spaces.defaultSolution to be configured via docker (#236570) (#236598)

# Backport

This will backport the following commits from `main` to `9.1`:
- [Allow xpack.spaces.defaultSolution to be configured via docker
(#236570)](#236570)

<!--- Backport version: 9.6.6 -->

### Questions ?
Please refer to the [Backport tool
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Co-authored-by: Larry Gregory <larry.gregory@elastic.co>

Author: kibanamachine

src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker

@@ -414,6 +414,7 @@ kibana_vars=(
     xpack.securitySolution.packagerTaskInterval
     xpack.securitySolution.prebuiltRulesPackageVersion
     xpack.spaces.maxSpaces
+    xpack.spaces.defaultSolution
     xpack.task_manager.capacity
     xpack.task_manager.claim_strategy
     xpack.task_manager.auto_calculate_default_ech_capacity