Upgrade framework-arduino-megaavr-megatinycore to 2.6.5

[mairas]

Upstream megaTinyCore has significant improvements and bugfixes. Could you please upgrade framework-arduino-megaavr-megatinycore?

Thanks! 😊

[SpenceKonde]

@mairas 2.6.4? sometime in the early morning on monday 2.6.5 mTC along with 1.5.4 DxC was released (not sure if DxC is in, but really hoping it is, because the Dx-series is a BFD.

Right now, to the best of my knowledge, both of the cores are in pretty good shape, and I don't forsee anything big in the next few months, so this would be a good time to do it,

[mairas]

Cool! Didn't visit the page before opening this issue, so my info was a couple of days too old. :-D Updated the request title.

[valeros]

Resolved in https://github.com/platformio/platform-atmelmegaavr/commit/89c1b5cbcdba7cbf5895a3c506a6822146780120