replace old Ethernet library by a submodule ref'ing the updated one

[d-a-v]

#5749 (comment)

[d-a-v]

We don't need it locally because

• we don't use it in examples
• we can't maintain it
• submoduling it will need regular updates following upstream (library manager is here for that)
• we can ship it in releases like we were doing with softwareserial before it was submoduled

To be noted: softwareserial is submoduled because we use it in examples. It is an esp8266 specific library so the author is also supposed to care about this (unlike Ethernet which is more arduino generic).

edit: we can also download softwareserial in CI, remove it as a submodule, and let users handle it with the library manager.

Comments are welcome.

[d-a-v]

Comments are welcome.

This library is not available from library manager (is it?)

Anyway comments came through the arduino maintainers chat board: We'd better have it locally instead of

• letting users unsuccessfully look for where it was before
• embed it in releases while not being available in git (like SoftwareSerial was)
• point to a known compatible version thanks to submodule git refs

(edit: PR will fail because of arduino API compatibility and is now moved inside fixing/pending #5969)