Regression in automatic base64 content encoding in v16

When applying gitlab_repository_file in version 16.0.3 we cannot use unencoded content anymore.

This is the new error that we see:

Error: Invalid base64 string in "content". Ensure the content is base64 encoded, or use the "base64encode" terraform function to encode it.

Relevant issues: #956 (closed)

This might be relevant to this MR: !1530 (merged)