libheif: update to 1.21.2

[brunvonlope]

Description

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 26.3.1 25D2128 arm64
Xcode 26.4 17E192

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install? (doesn't work on port bumps)
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken? (tested rav1e)

[macportsbot]

Notifying maintainers:
@mascguy for port libheif.
@MarcusCalhoun-Lopez for port libheif.