[Testing] Consider using a testing versions file similar to Elastic Agent

When Fleet Server's version is updated after a release (e.g. https://github.com/elastic/fleet-server/pull/3503), there could be a period of time when stack products of the updated version are not yet available for Fleet Server to use in testing. During this period of time, Fleet Server tests relying on these product versions will fail.

Elastic Agent faced a similar issue and it was decided to resolve it by creating a versions file in the Agent repo and use it do define the list of stack versions that Agent could test against. See:
* https://github.com/elastic/elastic-agent/issues/4296
* https://github.com/elastic/elastic-agent/blob/main/.agent-versions.json
* https://github.com/elastic/elastic-agent/blob/main/.github/workflows/bump-agent-versions.yml
* https://github.com/elastic/elastic-agent/blob/main/.github/workflows/bump-agent-versions.sh

# CI failures examples:
- https://github.com/elastic/fleet-server/pull/3476 Bumping version from 8.14.0 to 8.14.1 (8.14.1 snapshot not being available at this time)
- https://buildkite.com/elastic/fleet-server/builds/3917#018f3fb8-2661-483a-9e18-f29c9caa8687:~:text=%E2%94%82%20Error%3A,0%5D.elasticsearch.version)
Error:
```
│ Error: failed creating deployment
│
│ with ec_deployment.deployment,
│ on main.tf line 46, in resource "ec_deployment" "deployment":
│ 46: resource "ec_deployment" "deployment" {
│
│ api error: 1 error occurred:
│ * clusters.cluster_invalid_plan: Version [8.14.1-SNAPSHOT] is not available. Please use one of the available versions (resources.elasticsearch[0].elasticsearch.version)
```