sysutils/conky: update to 1.21.9

ChangeLog: https://github.com/brndnmtthws/conky/releases/tag/v1.21.9

Features

• Implement -U for FreeBSD.
• OpenBSD: Fill missing fields in get_top_info().
• Implement -U for Haiku.

Bug fixes

• lua-imlib2: fix memory leak
• Fix OpenBSD build
• Fix NetBSD build
• OpenBSD: Fix CPU Usage.

Miscellaneous

• Bump version
• build(deps): bump DeterminateSystems/nix-installer-action from 15 to 16 in the actions group
• Remove macOS-12 build (deprecated)