meson: Use implicit builtin dirs in pkgconfig generation

meson: Use implicit builtin dirs in pkgconfig generation

Starting with Meson 0.62, meson automatically populates the variables
list in the pkgconfig file if you reference builtin directories in the
pkgconfig file (whether via a custom pkgconfig variable or elsewhere).
We need this, because ${prefix}/libexec is a hard-coded value which is
incorrect on, for example, Debian.

Bump requirement to 0.62, and remove version compares that retained
support for older Meson versions.

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1245

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3061>