Bump the cache version to 8

Bump the cache version to avoid unexpected behavior in
older version of fontconfig.

There was no real incompatible features introduced in a cache.
But the older version of fontconfig can't deal with variable fonts
and then it failed. To avoid this breakage, you may want to choose
one of options like:

1) Applying this change to fontconfig in host and remove all the
fontconfig caches and then regenerate them by fc-cache linked with
older one and new one. or simply run applications.
You'll see two versions of caches in the cache directories.

2) Remove variable fonts on system and remove all the fontconfig
caches too. then regenerate caches. You'll still see one version of
caches there but without the entries of variable fonts.

After that, the problem should be gone.

https://bugzilla.redhat.com/show_bug.cgi?id=1981500
11 jobs for bump-cache-8 in 7 minutes and 27 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Build
passed meson android arm64 fedora #13048803

00:01:30

passed meson macos #13048801
gst-macos-10.15

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passed meson-mingw-w64-build #13048798

00:05:50

manual meson msys2 #13048802
docker windows 1809 allowed to fail manual
passed meson-shared-build #13048796

00:05:46

passed meson-static-build #13048797

00:06:27

passed meson vs2017 amd64 #13048799
docker windows 1809

00:01:17

passed meson vs2017 x86 #13048800
docker windows 1809

00:01:16

passed mingw-build #13048795

00:06:46

passed shared-build #13048793

00:07:27

passed static-build #13048794

00:06:55