add some #[must_use] to LockedAxes builder fns

[RJ]

Objective

• Helps avoid my mistake of grabbing LockedAxes from a query and calling unlock_rotation() without reassigning to the component, failing to notice it returns self

Solution

• Adds #[must_use] to builder-style fns that return Self, yielding this compiler warning:

warning: unused return value of `avian2d::prelude::LockedAxes::unlock_rotation` that must be used
  --> crates/avian2d/examples/uncommitted_example_fixed_joint_2d.rs:89:17
   |
89 |                 locked_axes.unlock_rotation();
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_must_use)]` on by default
help: use `let _ = ...` to ignore the resulting value
   |
89 |                 let _ = locked_axes.unlock_rotation();
   |                 +++++++