Lintian tag information: upstream-metadata-not-yaml-mapping (type: warning)

Description (from lintian-explain-tags)

The DEP 12 metadata file is not well formed. The document level must be a
YAML mapping:

Some-Field: some-value
Another-Field: another-value

Sometimes, the fields are mistakenly prefaced with a hyphen, which makes
them a YAML sequence. In that case, please remove the hyphens.

Please refer to https://dep-team.pages.debian.net/deps/dep12/ for details.

Visibility: warning
Show-Always: no
Check: debian/upstream/metadata

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