Fix some issues reported by lintian

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)
• Update standards version to 4.6.1, no changes needed. (out-of-date-standards-version)

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Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/btest/4485e611-88e7-49e5-bf04-791acd4a6acb.

Debdiff

These changes affect the binary packages:

File lists identical (after any substitutions)

Control files: lines which differ (wdiff format)

• Homepage: [-http​://www.zeek.org/-] {+https​://www.zeek.org/+}

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