chore(semantic-release): Print a deprecation warning when `$SEMANTIC_RELEASE_GITLAB_TOKEN` is unset.

Previously, if a user provided the `vault` input, it was ignored if either the
`$SEMANTIC_RELEASE_GITLAB_TOKEN` or `$GITLAB_TOKEN` variable was set. With this
change, we'll always use the token from Vault if explicitly provided by the
user.

Issue: gitlab-com/gl-infra/common-ci-tasks#15

… when deciding between Vault and CI variable.

Issue: gitlab-com/gl-infra/common-ci-tasks#15

The previous check, `$SEMANTIC_RELEASE_GITLAB_TOKEN != ""` appears to be
interpreted as "not defined or not empty" (i.e. the inversion of "defined and
empty"), which returns `true` if the variable is not defined. That is not the
intention here.

Issue: gitlab-com/gl-infra/common-ci-tasks#15

Previously, the `semantic_release` job required its token to be stored in a CI
variable. With this change, the token can optionally be stored in Vault
instead, while keeping the previous behavior as the default.