Update --features flags and enable StableScheduling by default

[cofyc]

What problem does this PR solve?

Enable StableScheduling by default.

What is changed and how does it work?

• Update feature gates to support key={true,false} syntax
  • This is required to disable a feature e.g. StableScheduling=false.
• Enable StableScheduling by default

Check List

Tests

• Unit test
• E2E test
• Stability test

Code changes

• Has Helm charts change
• Has Go code change
• Has documents change

Side effects

• Should pass =true

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Does this PR introduce a user-facing change?:

ACTION REQUIRED: tidb-scheduler --features flag has been updated to use `key={true,false}` format, please append `=true` to enable the feature.

[cofyc]

/run-e2e-tests