Usage of `flushregs` in gdb will result in the following warning:
```
Warning: 'flushregs', an alias for the command 'maintenance flush register-cache', is deprecated.
Use 'maintenance flush register-cache'.
```
This patch updates to use the recommend command.
Signed-off-by: Daniel Mangum <georgedanielmangum@gmail.com>
Merges https://github.com/espressif/esp-idf/pull/11535