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GH-45118: [Packaging] Use armored keyring for APT repository (#45131)

### Rationale for this change

We're using the keybox format for keyring because we have multiple GPG keys. It seems that we should use the armored format for keyring.

libapt-pkg 2.9.16 or later shipped by Debian trixie rejects the keybox format files. So we should use the armored format.

### What changes are included in this PR?

Use `.asc` not `.gpg` for keyring.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #45118

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>