Improving CONTRIBUTING.md for backport details (#23057)

See the changes.

Two key points:
* Necessary enhancements could be backported.
* The backports shouldn't break downgrade  between minor releases.

CONTRIBUTING.md

@@ -299,9 +299,7 @@ known as the release freeze. All the feature pull requests should be
 merged before feature freeze. And, during the frozen period, a corresponding
 release branch is open for fixes backported from main branch. Release candidates
 are made during this period for user testing to
-obtain a final version that is maintained in this branch. A release is
+obtain a final version that is maintained in this branch.
-maintained by issuing patch releases to only correct critical problems
-such as crashes or security issues.
 
 Major release cycles are seasonal. They always begin on the 25th and end on
 the 24th (i.e., the 25th of December to March 24th).
@@ -311,6 +309,16 @@ for the previous version. For example, if the latest, published release is
 v1.2, then minor changes for the previous release—e.g., v1.1.0 -> v1.1.1—are
 still possible.
 
+The previous release gets fixes for:
+
+- Security issues
+- Critical bugs
+- Regressions
+- Build issues
+- Necessary enhancements (including necessary UI/UX fixes)
+
+The backported fixes should avoid breaking downgrade between minor releases as much as possible.
+
 ## Maintainers
 
 To make sure every PR is checked, we have [team