Integrate gitlab-ci/hide-stages-if-variables-not-set and 3 more

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@89fbce24

Hide Confluence stage if confluence variables not set

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@db50e93b

Hide deploy stage if DEPLOY_RELEASE_TARGET not set

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@f4aa0048

Hide FTP stage if DEPLOY_FTP_TARGET not set

This makes the variable HIDE_FTP_UPLOAD_STAGE obsolete, because we can
hide the FTP stage now by setting DEPLOY_FTP_TARGET to an empty string.

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Commit: seco-ne/yocto/infrastructure/gitlab-ci@409c4f27

Do not run CI test sub-pipeline on master