gitlab: unify treatment of variables fields

[chemamartinez]

Proposed commit message

This fixes the treatment of fields:

• gitlab.production.params.variables
• gitlab.production.graphql.variables
• gitlab.production.params.graphql.variables

I wrongly assumed that they should contain always an object as the docs suggested, and that the [FILTERED] string value that test logs contain was the result of obfuscation during the development.

It turns out that these string values come from the GitLab API so further changes are required to always treat these fields as keyword.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)

[elastic-vault-github-plugin-prod]

🚀 Benchmarks report

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[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 928f6da

cc @chemamartinez

[elastic-vault-github-plugin-prod]

Package gitlab - 2.3.3 containing this change is available at https://epr.elastic.co/package/gitlab/2.3.3/