Support Git LFS 3.0 clients

The Git LFS 3.0.0 client is due to release Friday, September 24, 2021. It contains a number of backwards incompatible changes (See: https://github.com/git-lfs/git-lfs/pull/4629)

Initial testing of the pre-release version finds only one problem; the git-lfs-authenticate SSH command fails due to the change in arguments described here:

When using an SSH URL (that is, the syntax starting with `ssh://`), the leading slash is not stripped off when invoking `git-lfs-authenticate` or `git-lfs-transfer`. This is compatible with the behavior of Git when invoking commands over SSH.

As a result Git LFS 3.0 clients when using SSH will fail like so (due to the inclusion of the leading slash):

Error downloading object: galaxy1.jpg (4fa32d6): Smudge error: Error downloading galaxy1.jpg (4fa32d6f9b1461c4a53618a47324e243e36ce7ceae72ad440cc811a7e6881be3):
batch request: Bitbucket cannot execute 'git-lfs-authenticate /myproject/myrepo.git download'. The command is not supported as entered.

Note: The scope of this issue only includes support for the existing/required functionality that currently works with 1.x and 2.x clients. Support for new and optional functionality will be tracked elsewhere; specifically support for the pure SSH protocol is tracked here: https://jira.atlassian.com/browse/BSERV-12990