Allowing the gitlab token to be 20 characters or more

[jmini]

Fixes #207

As discussed in #210 (comment), since the prefix can be glpat- or anything (from empty String to anything more) see https://docs.gitlab.com/ee/user/admin_area/settings/account_and_limit_settings.html#personal-access-token-prefix

The check should be changed to accept 20 characters or more.

I reworked commit proposed by @sebastiangraf (added as co-author of my commit) in #210

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[jetersen]

Released in https://github.com/jenkinsci/gitlab-branch-source-plugin/releases/tag/636.v55fd8144d335