Add CHANGELOG entries that were forgotten in the past

CHANGELOG.md

+## [0.2.7] - 2021-04-21
+
+* Update ONLYOFFICE version to 6.2.2.21
+
+## [0.2.6] - 2021-03-18
+
+* Allow USE_UNAUTHORIZED_STORAGE and ONLYOFFICE_HTTPS_HSTS_ENABLED
+
+## [0.2.5] - 2021-03-18
+
+* Specify proper repo URLs, add Chart.lock
+* Deprecate @stable repo
+* Small fixes
+
 ## [0.2.4] - 2020-07-08
 
 * Update configuration of sociallogin plugin to work with newest version of app