licensing: relicense C++ bindings under LGPL-2.1-or-later

licensing: relicense C++ bindings under LGPL-2.1-or-later

Commit ea84f882d5d3 ("licensing: relicense C++ library code under
LGPL-3.0-or-later") changed the license of C++ bindings in order to
solve potential issues with code generated from templates[1], default
implementations, etc. However this change makes the bindings less
attractive to projects that have strict licensing restrictions and avoid
GPL-3.0 code[2].

After talking to Grant Likely I decided that the best approach is to
make the bindings available under LGPL-v2.1-or-later and simply let the
end user decide which version's text to apply.

While at it: tweak the README to also mention that examples are provided
under GPL-2.0-or-later.

Link: [1] https://www.spinics.net/lists/linux-gpio/msg46605.html
Link: [2] https://github.com/brgl/libgpiod/issues/72
Suggested-by: Walter Lozano <walter.lozano@collabora.com>
Suggested-by: Grant Likely <grant.likely@linaro.org>
Acked-by: Kent Gibson <warthog618@gmail.com>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Link: https://lore.kernel.org/r/20240416212141.6683-2-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>