Update working version to Babeltrace 2.1.0-rc1

Update working version to Babeltrace 2.1.0-rc1

This is not the _release_ commit of Babeltrace 2.1.0-rc1; it merely
updates the current working version to 2.1.0-rc1.

Since this commit is not tagged, the `git describe`
output (e.g. v1.2.0-3402-g836e3fd0d) will be included as part of the
library's "development stage" version field.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ia868892bd611ff11bba5b01399a78f6cb5c7f6c2
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2845
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>