Use secure URI in Homepage field

Fix some issues reported by lintian

• Use secure URI in Homepage field. (homepage-field-uses-insecure-uri)
• Re-export upstream signing key without extra signatures. (public-upstream-key-not-minimal)

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Build and test logs for this branch can be found at https://janitor.debian.net/run/c7a5794a-8aa7-438a-8c82-f367a23b25fd.