textadept: update to 12.9

[skinchangerdev]

See #60241

This PR updates Textadept to 12.9. Note that this update includes significant changes, such as:

• A switch to CMake from Unix Makefiles
• A new QT version, which is the default (the GTK version is still available as textadept-gtk)

Testing the changes

• I tested the changes in this PR: YES

Local build testing

• I built this PR locally for my native architecture, (x86_64)
• I built this PR locally for these architectures:
  • aarch64

[skinchangerdev]

The failed CI check is due to it being unable to fetch an archive after a connection times out and is not intrinsic to this PR. It should succeed if reran.

[skinchangerdev]

Well. After someone reran the CI the x86_64 glibc native build failed again with the exact same error. I'm beginning to suspect the CI environment for x86_64 native builds is broken in some way, given that all of the x86_64 native builds on my machine have worked flawlessly.