Prepare devtools_shared for publishing to version 12.1.0

[elliette]

Work towards #9633

The following changes have been made to devtools_shared since we last published version 12.0.0:

04b74a8 Add webdriver test for compiler query parameter (#9608)
976bd6a Fixes null error when parsing universal link settings (#9581)
98a52a8 Add feature flag scaffolding to support experiments restricted to a specific Flutter channel (#9440)
5e17922 (tag: v2.50.0) Revert PR #9372: Un-delete packages/devtools_shared/lib/src/test/chrome.dart (#9409)
e918e9f Get DevTools tests passing with latest Flutter candidate (#9382)
bbc7cbc Delete unused test utilities file chrome.dart (#9372)
0ac0c92 (cpu-compare) Update to latest Flutter candidate version (#9351)
318dbcf Rework the integration test timeout logic (#9322)
879d56e Add timestamps to various test logging utilities (#9319)
5812bd1 Add support for connecting to the DevTools server from integration tests (#9190)

This PR makes the breaking changes introduced in #9581 and #9319 non-breaking so that we can do a minor instead of major version bump. It also adds CHANGELOG entries for the above changes.

Confirmed non-breaking status with:

dart-apitool diff \
  --old "git://https://github.com/flutter/devtools/packages/devtools_shared:c65f3f2e353750614b66c664d4dd451853d1035e" \
  --new "git://https://github.com/elliette/devtools/packages/devtools_shared:prepare-devtools-shared-for-publish" \
  --force-use-flutter

Note: Opened #9650 so that we catch missing CHANGELOG entries and breaking changes in the future.