samples: net: socket: echo_server: Restore generic openthread sample

Since multiple platforms than nrf are allowed to run this sample,
re-add generic openthread name for sample.

Initially, generic naming has been performed in #83195,
and then, a revert occurred in #87269

Signed-off-by: Cristian Bulacu <[email protected]>

Author: Cristib05

samples/net/sockets/echo_server/sample.yaml

@@ -83,7 +83,7 @@ tests:
     tags:
       - net
       - usb
-  sample.net.sockets.echo_server.nrf_openthread:
+  sample.net.sockets.echo_server.openthread:
     extra_args: EXTRA_CONF_FILE="overlay-ot.conf"
     slow: true
     tags: