Deprecate `--system` and `--no-system` in `uv venv` (#5925)
e.g.
```
❯ cargo run -- venv --no-system
Blocking waiting for file lock on build directory
Compiling uv v0.2.34 (/Users/zb/workspace/uv/crates/uv)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 19.85s
Running `target/debug/uv venv --no-system`
warning: The `--no-system` flag has no effect, a system Python interpreter is always used in `uv venv`
Using Python 3.12.4 interpreter at: /opt/homebrew/opt/python@3.12/bin/python3.12
Creating virtualenv at: .venv
Activate with: source .venv/bin/activate
❯ cargo run -- venv --system
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.15s
Running `target/debug/uv venv --system`
warning: The `--system` flag has no effect, a system Python interpreter is always used in `uv venv`
Using Python 3.12.4 interpreter at: /opt/homebrew/opt/python@3.12/bin/python3.12
Creating virtualenv at: .venv
Activate with: source .venv/bin/activate
```
