Set default strict_optional state to True

[hauntsaninja]

No description provided.

[hauntsaninja]

Looks like some of the strict optional behaviour is a little buggy, e.g. not symmetric join / meet

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[cdce8p]

Should we consider removing the --implicit-optional and --no-strict-optional options and related code all together, maybe for the 2.0 release (#17967)?

--implicit-optional was disabled by default in 0.980.

[JukkaL]

We still use --no-strict-optional for some legacy code at work. Migrating away from it can be tricky, so we may have to keep it around for a long time. --implicit-optional doesn't need much maintenance, so I think we can also keep it around. It's hard to know how popular various flags are in private codebases.