fix: Replace deprecated flushregs alias with full gdb command

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

examples/system/sysview_tracing/pytest_sysview_tracing.py

@@ -22,7 +22,7 @@ def test_examples_sysview_tracing(dut: IdfDut) -> None:
         dut.expect(re.compile(rb'example: Task\[0x3[0-9A-Fa-f]+\]: received event \d+'), timeout=30)
 
     dut.gdb.write('mon reset halt')
-    dut.gdb.write('flushregs')
+    dut.gdb.write('maintenance flush register-cache')
     dut.gdb.write('b app_main')
 
     dut.gdb.write('commands', non_blocking=True)