Several helper scripts assumes bash is available at /bin/bash. That breaks on systems
such as NixOS, where bash is resolved from PATH instead of a fixed /bin location and
causes failures like `bad interpreter` during `make format`, e.g., on my host machine:
```sh
$ make format
/PX4-Autopilot/Tools/astyle/check_code_style.sh: /PX4-Autopilot/Tools/astyle/fix_code_style.sh: /bin/bash: bad interpreter: No such file or directory
```
This change switches these entrypoints to `#!/usr/bin/env bash` so they locate bash properly.
No functional changes intended.
Signed-off-by: Onur Özkan <work@onurozkan.dev>
- use `gazebo-classic` everywhere consistently referring to the original Gazebo (eg version 9,10,11)
- additional `gazebo_*` helper targets added for compatibility, but warn about deprecation and tell you the new target naming
- use `gz` everywhere when referring to Gazebo (aka Ignition Gazebo or new Gazebo)
