Bump version to 13.0.50.DATE-git.

Bump version to 13.0.50.DATE-git.

Now that the GDB 12 branch has been created,
this commit bumps the version number in gdb/version.in to
13.0.50.DATE-git

For the record, the GDB 12 branch was created
from commit 2be64de603f8b3ae359d2d3fbf5db0e79869f32b.

Also, as a result of the version bump, the following changes
have been made in gdb/testsuite:

	* gdb.base/default.exp: Change $_gdb_major to 13.