Telegram's Bot API only allows a specific set of emoji for bot reactions
(the ReactionEmoji enum). ✅ (U+2705) and ❌ (U+274C) are not in that
set, causing on_processing_complete reactions to silently fail with
REACTION_INVALID (caught at debug log level).
Replace with 👍 (U+1F44D) / 👎 (U+1F44E) which are always available in
Telegram's allowed reaction list. The 👀 (eyes) reaction used by
on_processing_start was already valid.
Based on the fix by @ppdng in PR #6685.
Fixes#6068