Cover C++20 in CI (fixing char8_t-related problem there).

[anforowicz]

This is the CI coverage of C++20 (and other C++ versions) that was discussed in #1432. Let me start as a "draft" PR, to quickly check if the new CI steps actually pass.

[anforowicz]

Okay, I think this is ready for a review - can you PTAL?

(I see that it would have detected a regression that I tried introducing in #1438 as an ad-hoc test.)

[anforowicz]

Oh, but I guess now I need to check why C++11 fails...

[anforowicz]

Hmmm... I guess C++11 is just not supported at this point - e.g.:

 cargo:warning=src/blobstore.cc: In member function 'org::blobstore::BlobMetadata org::blobstore::BlobstoreClient::metadata(uint64_t) const':
  cargo:warning=src/blobstore.cc:61:23: error: use of 'auto' in lambda parameter declaration only available with '-std=c++14' or '-std=gnu++14'
  cargo:warning=   61 |                   [&](auto &t) { metadata.tags.emplace_back(t); });
  cargo:warning=      |                       ^~~~

[dtolnay]

Thanks!