Allow snapshot stack versions to query EPR for serverless

[mrodm]

This PR allows to use stack SNAPSHOT versions. In order to query to EPR (Elastic Package Registry), it removes the SNAPSHOT prefix for the kibana.version query parameter.

Currently, if a SNAPSHOT version is used , for instance 8.11.0-SNAPSHOT, elastic-package fails creating the agent policy, because it does not find any system package:

 $ elastic-package stack up -v -d --version 8.11.0-SNAPSHOT --provider serverless
...
2023/10/03 18:28:05  INFO Creating agent policy
Error: booting up the stack failed: failed to create agent policy: unexpected number of system package versions - found 0 expected 1

[elasticmachine]

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• 💔 Build #1650 failed edcb26c

cc @mrodm