Add deprecation notice to `inputs.leofs` documentation

[Hipska]

Feature Request

The inputs.leofs will become deprecated (see #9478), so the docs of this plugin should describe this to the users.

Proposal:

Add example configuration how to achieve the same with inputs.snmp.

[sjwang90]

@Hipska It would be nice to target this deprecation announcement for the next minor release (1.21 in December) along with all the new deprecation changes.