Add changelog for 280
This MR was automatically created from CI, https://gitlab.com/gitlab-org/gitter/gitter-android-app/-/jobs/141678302
We copied CURRENT_VERSION.txt
-> 280.txt
This MR was automatically created from CI, https://gitlab.com/gitlab-org/gitter/gitter-android-app/-/jobs/141678302
We copied CURRENT_VERSION.txt
-> 280.txt