gitlab-ci: drop gitlint and use generalize by using environment variables

[mniestroj]

gitlint is already part of Docker image, so there is no need to install it manually.

There is already $CI_DEFAULT_BRANCH provided by GitLab CI, so use it instead of hardcoded main.

Introduce global $MODULE_PATH, so that modules/lib/golioth does not need to be hardcoded in multiple places.

[github-actions[bot]]

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