Link check on REFERENCE_LINK field of keywords.txt not working with Arduino IDE >1.8.15

[per1234]

The check_keywords_txt function has an optional feature to check whether there is a reference page for the values in the REFERENCE_LINK field of keywords.txt.

Arduino IDE 1.x has a Help > Find in Reference feature where the reference page for the selected keyword is opened in the browser. In Arduino IDE 1.8.15 and older, a bundled copy of the reference pages was used to allow this feature to work regardless of whether there was an Internet connection. The system used to generate that reference content was broken during the migration to the current Arduino Language Reference content system. Since the offline reference content was no longer possible to maintain, the reference lookup feature was changed to simply open a search for the keyword on search.arduino.cc (arduino/Arduino#11495).

Since the new system draws from the entire contents of arduino.cc, including the forum, there is no way for the reference page check of check_keywords_txt to validate whether relevant information will be returned by the Help > Find in Reference feature of Arduino IDE >1.8.15.

The check still has some value for validating the values for compatibility with the older versions of the Arduino IDE, so I don't see a need to remove it. However, it will fail spuriously when a version of the Arduino IDE >1.8.15 was set via install_ide before running the function. The check must be skipped if a later version is used, with a warning printed that clearly communicates the limitation to the user.