Added release notes for 2.21.4 release.

[noissue]

(cherry picked from commit f064303)

Pulp 2.21.3 GA

Pulp 2.21.2 GA

pulp 2.21.2 beta 1

Merge pull request pulp#3978 from evgeni/2.21.1-ga

Pulp 2.21.1 GA

Merge pull request pulp#3977 from evgeni/2.21.1-beta

pulp 2.21.1 beta 1

Pulp 2.21.0

Pulp 2.21.0 RC1

Pulp 2.20.1

Pulp 2.20.1 rc 1