HPCC-8951 Incorrect extension for proxy plugins

Note that there is also a corresponding change in LN repo.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4088 from richardkchapman/proxy-plugins-rename

HPCC-8951 Incorrect extension for proxy plugins

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>