Prepare for minor release/2.6 (#559)

Signed-off-by: sagargurung1001@gmail.com <sagargurung1001@gmail.com>

.github/workflows/nighlty-ci-release-branch.yml

@@ -9,4 +9,4 @@ jobs:
     uses: ./.github/workflows/shared_workflow.yml
     secrets: inherit
     with:
-      branch: release/2.5
+      branch: release/2.6

.tx/backport

@@ -4,3 +4,4 @@ release/2.2
 release/2.3
 release/2.4
 release/2.5
+release/2.6

CHANGELOG.md

@@ -6,10 +6,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
-## 2.6.0-20240117-nightly - 2024-01-15
+## 2.6.0 - 2024-01-17
 ### Changed
 - This release expects OpenProject version 13.2 or newer
-- [What's Changed](https://github.com/nextcloud/integration_openproject/releases/tag/v2.5.1)
+- [What's Changed](https://github.com/nextcloud/integration_openproject/releases/tag/v2.6.0)
 
 ## 1.0.5 – 2021-06-28
 ### Changed