Set minimum HarfBuzz version back to 2.0.0.

Due to the new GSUB parsing we no longer need
`hb_ot_layout_lookup_get_glyph_alternates`.

This partially reverts commit 5d2fd7608a9.

* CMakeLists.txt (HARFBUZZ_MIN_VERSION), meson.build (harfbuzz_dep),
  builds/unix/configure.raw (harfbuzz_pkg): Set version to 2.0.0.

* src/autofit/ft-hb-decls.h: Remove entry for
  `hb_ot_layout_lookup_get_glyph_alternates`.
5 files changed