Bump version to 17.0.50.DATE-git.

Now that the GDB 16 branch has been created,
this commit bumps the version number in gdb/version.in to
17.0.50.DATE-git

For the record, the GDB 16 branch was created
from commit ee29a3c4ac7adc928ae6ed1fed3b59c940a519a4.

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 17.