Deprecate instance name argument in favor of flag

Per the team call:
- Deprecate the passing of an instance name argument (`edgedb instance stop myinst`) in favor of a flag (`edgedb instance stop -I myinst`). Support for passing an instance name argument will be removed in a future release.
- Running naked instance commands (`edgedb instance stop`) should run against any project-linked instances if one exists.
- Passing any other connection flag other than `-I` should produce an error message explaining that only `-I/--instance` is supported for `edgedb instance` commands.

Start at the CLI entry point for the `edgedb instance` commands, especially `edgedb instance stop`. Check how positional instance names, `-I/--instance`, project-linked instances, and other connection flags are currently handled. Done means the legacy argument is deprecated, naked commands use a linked instance when available, and unsupported connection flags produce the specified error.

Written by the indexing model from the issue text.