Add missing entry to changelog and release-notes of 2.6.0 (fixes #1307)

[sequba]

Context

Hotfix for #1307

Types of changes

• [ ] Breaking change (a fix or a feature because of which an existing functionality doesn't work as expected anymore)
• [ ] New feature or improvement (a non-breaking change that adds functionality)
• [x] Bug fix (a non-breaking change that fixes an issue)
• [ ] Additional language file, or a change to an existing language file (translations)
• [ ] Change to the documentation

[github-actions[bot]]

Performance comparison of head (7eb5f6b3d29599e85d0cc0c9879c98684d61e5ae) vs base (c4b6bc392fb38aa24d142811bcbc594e881aa4df)

                                     testName |   base |   head |  change
-------------------------------------------------------------------------
                                      Sheet A | 759.03 | 748.08 |  -1.44%
                                      Sheet B | 256.66 | 254.58 |  -0.81%
                                      Sheet T | 218.05 | 218.64 |  +0.27%
                                Column ranges | 849.17 | 838.25 |  -1.29%
Sheet A:  change value, add/remove row/column |     57 |     55 |  -3.51%
 Sheet B: change value, add/remove row/column |    405 |    403 |  -0.49%
                   Column ranges - add column |    272 |    240 | -11.76%
                Column ranges - without batch |    630 |    826 | +31.11%
                        Column ranges - batch |    347 |    161 | -53.60%