Hotfix: 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 (84e260034eaf65c6cdb0e6518053a185b2e3e9db)

                                     testName |   base |   head |   change
--------------------------------------------------------------------------
                                      Sheet A | 713.42 | 716.49 |   +0.43%
                                      Sheet B | 227.25 | 228.93 |   +0.74%
                                      Sheet T | 205.54 | 203.86 |   -0.82%
                                Column ranges | 782.18 | 781.13 |   -0.13%
Sheet A:  change value, add/remove row/column |     52 |     47 |   -9.62%
 Sheet B: change value, add/remove row/column |    360 |    345 |   -4.17%
                   Column ranges - add column |    203 |    193 |   -4.93%
                Column ranges - without batch |    719 |    542 |  -24.62%
                        Column ranges - batch |    121 |    265 | +119.01%