Update UPGRADE-2.6.md
minor #12458 Update UPGRADE-2.6.md (perajovic)

This PR was submitted for the master branch but it was merged into the 2.6 branch instead (closes #12458).

Discussion
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Update UPGRADE-2.6.md

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | -
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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450e341 Update UPGRADE-2.6.md