Remove analyzer's deprecated analysis modes: no-implicit-casts and no-implicit-dynamic

[srawlins]

When these are removed, we can close issues related to them:

• Ignore variables named _, __, ... even when implicit-dynamic:false #26915
• Map literal type arguments sometimes aren't inferred with --no-implicit-dynamic #31013
• Analyzer with implicit-dynamic turned off fails to flag typedef #37437
• strong-mode documentation points to archived repo #48617
• documentation for strong-mode rules #48650

The new 'language' modes are documented here: https://dart.dev/guides/language/analysis-options#enabling-additional-type-checks

[bwilkerson]

Do we want to set a milestone for this?

[srawlins]

Good idea. Just set to Dart 3 Alpha.

[parlough]

I completely missed this somehow, but it looks like this removal was reverted in 94f20d7. Perhaps reopen this so the removal doesn't get lost?

Do you have any extra context on why it was reverted?

[srawlins]

I think it was removed because some flutter customer tests referenced them.