Bump \fmtversion to 2021/06/01 to satisfy memoir class requirements

[utkuokur]

This PR updates the hard-coded LaTeX format date in
lib/LaTeXML/Engine/latex_base.pool.ltxml:

• DefMacroI('\fmtversion', undef, '2018/12/01');

• DefMacroI('\fmtversion', undef, '2021/06/01');

Rationale:

• The memoir class (and some other modern packages) require at least LaTeX format 2021/06/01.

• With the previous value (2018/12/01), LaTeXML fails immediately with:

  Class memoir Error: Your LaTeX release is too old.
  The memoir class requires at LaTeX format from at least 2021/06/01 onwards.

• After applying this patch locally, I was able to successfully process documents using memoir with LaTeXML.

This should unblock users of memoir and other classes that check \fmtversion.

Notes:
While running make test, I observed 3 failing subtests in t/81_babel.t.
However, the same failures occur when testing against the current master branch, so they appear unrelated to this change.

[dginev]

That is fine by me, thanks.

But curious if Bruce has some thoughts about the broader impact of that flag, for example if it affects our "make formats" target.

[xworld21]

Format 2020 includes stuff like \NewDocumentCommand which LaTeXML does not implement directly, so it would break LaTeX3 (e.g. xparse would not load correctly). Rather, does make formats update \fmtversion?

[utkuokur]

The pull request failed the test. I will retry with a partial solution instead of a global change.

[dginev]

@utkuokur your PR was healthy, the failed test was due to CI bitrot (fixed in PR #2653 )