[deps] rev markdown, source_span, string_scanner
[deps] rev markdown, source_span, string_scanner

Revisions updated by `dart tools/rev_sdk_deps.dart`.

markdown (https://github.com/dart-lang/markdown/compare/93d0eee..9b61871):
  9b61871  2022-10-25  Kevin Moore  Make helper class private that should not have been exposed (#476)
  299964e  2022-10-26  le.chang  Return list for link nodes creation (#452)
  aee6a40  2022-10-25  Kevin Moore  validate code coverage on CI (#474)
  88f3f8a  2022-10-25  Zhiguang Chen  Fix html entity and numeric character references (#467)

source_span (https://github.com/dart-lang/source_span/compare/ff03af1..d1d47e5):
  d1d47e5  2022-10-25  Kevin Moore  Require Dart 2.18, update lints, latest pkg:lints, add dependabot (#88)

string_scanner (https://github.com/dart-lang/string_scanner/compare/2d84b16..10435a4):
  10435a4  2022-10-25  Kevin Moore  Update minimum SDK, latest pkg:lints, update CI actions (#48)

Change-Id: I94dbb0830600cb5304aaae16b2ae715bfc71411e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/265641
Auto-Submit: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@google.com>
Commit-Queue: Nate Bosch <dc212c74d8afde9c690fdd09e74d3e5c5d5a3de0@google.com>
Reviewed-by: Nate Bosch <dc212c74d8afde9c690fdd09e74d3e5c5d5a3de0@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk