Make the component compatible with IDFv4.1, v4.2, v4.3

components/esp_modem/examples/ap_to_pppos/README.md

@@ -15,3 +15,7 @@ By default, this example simply connects to the PPP server using a supported dev
 This example however, doesn't rely on sending specific AT commands, just the bare minimum to setup the cellular network.
 Thus, if the `EXAMPLE_USE_MINIMAL_DCE` option is enabled, we directly inherit from the `ModuleIf` and implement only the basic commands.
 Also, to demonstrate the dce_factory functionality, a new `NetDCE_Factory` is implemented for creating the network module and the DCE.
+
+### Supported IDF versions
+
+This example is only supported from `v4.2`, since is uses NAPT feature. 

components/esp_modem/examples/linux_modem/README.md

@@ -17,3 +17,7 @@ over PPPoS, i.e. over the modem serial line.
 * Experiment with the network, after getting the IP from the modem device
     - directly in the code
     - in the system (need to set `tun` interface IP, dns servers, and routing the desired traffic over the tun interface)
+
+### Supported IDF versions
+
+This example (using the default CMake IDF build system) is only supported from `v4.4`, since is uses `idf.py`'s linux target.  

components/esp_modem/examples/modem_console/README.md

@@ -14,4 +14,8 @@ This example implements two very simple network commands to demonstrate and test
 To demonstrate creating custom modem devices, this example creates a DCE object using a locally defined create method,
 that sets up the DCE based on a custom module implemented in the `my_module_dce.hpp` file. The module class only overrides
 `get_module_name()` method supplying a user defined name, but keeps all other commands the same as defined in the `GenericModule`
-class.
+class.
+
+### Supported IDF versions
+
+This example is only supported from `v4.2`, due to support of the console repl mode. 

components/esp_modem/examples/pppos_client/README.md

@@ -8,3 +8,7 @@ This example shows how to act as a MQTT client after the PPPoS channel created b
 ## How to use this example
 
 See the README.md file in the upper level `pppos` directory for more information about the PPPoS examples.
+
+### Supported IDF versions
+
+This example is only supported from `v4.1`, as this is the default dependency of `esp-modem` component. 

components/esp_modem/examples/simple_cmux_client/README.md

@@ -15,3 +15,7 @@ The example uses the following configuration options to demonstrate basic esp-mo
 ## About the esp_modem
 
 Please check the component [README](../../README.md)
+
+### Supported IDF versions
+
+This example is only supported from `v4.3`, since is uses an experimental `esp_event_cxx` component.