Update missing release notes in 1.20.3 (#4528)
Update missing release notes in 1.20.3 (#4528)

Motivation:

When the target release version changed from 1.21.0 to 1.20.3, some PRs were already merged into the master branch. So their milestone should be changed and the release notes also need to be updated.

Modifications:

- Add #4515, #4507, #4484 and #4492 to the release notes.

Result:

Update the missing issues and PRs.

line/armeria,line/armeria,line/armeria,line/armeria,line/armeria,line/armeria