Bump LTTNG_UST_ABI version from 8.1 to 9.0

Bump LTTNG_UST_ABI version from 8.1 to 9.0

Ensure an older liblttng-ust communicating with a newer liblttng-ust
will refuse the handshake, so older session daemon do not get to
interact with more recent lttng-ust which implements nested arrays and
sequences.

Introduce LTTNG_UST_ABI_MAJOR_VERSION_OLDEST_COMPATIBLE to allow
liblttng-ust to be backward compatible with a range of major versions.
In this case, liblttng-ust ABI 9.0 is fully backward compatible with
8.x, given that none of the existing ABI in 8.x has changed in 9.0.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>