Mark acra-backup deprecated

[Zhaars]

Checklist

• Change is covered by automated tests
• The coding guidelines are followed
• Public API has proper documentation in the Acra documentation site or has PR on documentation repository
  with new changes
• CHANGELOG.md is updated (in case of notable or breaking changes)
• CHANGELOG_DEV.md is updated
• Benchmark results are attached (if applicable)
• Example projects and code samples are up-to-date (in case of API changes)

[vixentael]

Are we sure that acra-keys should be used instead of acra-backup?

I don't see any backup command in the list of commands
https://docs.cossacklabs.com/acra/configuring-maintaining/general-configuration/acra-keys/

[Zhaars]

Are we sure that acra-keys should be used instead of acra-backup?

I don't see any backup command in the list of commands https://docs.cossacklabs.com/acra/configuring-maintaining/general-configuration/acra-keys/

acra-keys has export/import subcommands that preform keys backup.