Update source controller to improve managed transport

[pjbgf]

Stability and reliability improvements

• Retry on stale connections: fixes the issue of long-running connections resulting in EOF for GitLab servers. Although this may also affect other Git Servers in different scenarios.
• Handle closing of stale connections: dispose resources and remove connection from cache.
• Optimise mutex on cached connections for decreased waiting time when competing go-routines try to create a SSH connection.

Low Impact Breaking Changes

• HTTP(S) URLs must be 2048 characters long.
• SSH Path must be 4096 characters long.

xref: fluxcd/source-controller#658