Bump to platform 8.1

[danirabbit]

[ryonakano]

@danirabbit I modified this branch so that the CI succeeds.

Changes Summary

1. Update folks to 0.15.9

I fixed the Flatpak CI failing with the following error by updating folks to 0.15.9, which also matches the version that can be installed from apt.

../backends/dummy/lib/dummy-persona-store.vala: In function ‘folks_dummy_persona_store_update_is_user_set_default’:
../backends/dummy/lib/dummy-persona-store.vala:998:2: error: implicit declaration of function ‘folks_persona_store_set_is_user_set_default’; did you mean ‘folks_persona_store_get_is_user_set_default’? [-Wimplicit-function-declaration]
  998 |       this.is_user_set_default = is_user_set_default;
      |  ^    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |  folks_persona_store_get_is_user_set_default

2. Explicit libdir for folks

After 1, I faced the following error that Mail can't find folks on Flatpak x86_64 CI, so I specified the libdir for folks.

Run-time dependency folks found: NO (tried pkgconfig and cmake)

../meson.build:25:12: ERROR: Dependency "folks" not found, tried pkgconfig and cmake

A full log can be found at /run/build/mail/_flatpak_build/meson-logs/meson-log.txt
Error: module mail: Child process exited with code 1

[ryonakano]

I fixed Mail builds and runs successfully with Flatpak both on amd64 and arm64. Also bumped platform from 8.1 to 8.2.