package/{mesa3d, mesa3d-headers}: bump version to 21.0.2

Release notes:
21.0.0: https://lists.freedesktop.org/archives/mesa-announce/2021-March/000622.html
21.0.1: https://lists.freedesktop.org/archives/mesa-announce/2021-March/000624.html
21.0.2: https://lists.freedesktop.org/archives/mesa-announce/2021-April/000625.html

DRI swrast driver was removed:
https://cgit.freedesktop.org/mesa/mesa/commit/?h=21.0&id=435de835cd639d1b9bb96f81fc224771dc90af6d

OSMesa classic support was removed:
https://cgit.freedesktop.org/mesa/mesa/commit/?h=21.0&id=ee802372180a2b4460cc7abb53438e45c6b6f1e4



To avoid any conflict, and to show that the new OSMesa is Gallium-based,
we name the new option with a _GALLIUM suffix, even though this is now
the only OSMesa implementation left.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
[yann.morin.1998@free.fr:
  - rename the new option s/$/_GALLIUM/
  - don't drop the the old (pre-classic) legacy option
  - slightly reword the OSMesa help entry
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>