Feature: bump branch to 2.6

[SuZhou-Joe]

Description

In 2.5 we use #3274 to manually update pkg.branch.
We have raised #3483 to do it automatically but shall not be able to catch up with 2.6 release, so do it manually.

Issues Related

#3490

Check List

• All tests pass
  • yarn test:jest
  • yarn test:jest_integration
  • yarn test:ftr
• New functionality includes testing.
• New functionality has been documented.
• Update CHANGELOG.md
• Commits are signed per the DCO using --signoff

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (2.6@a4e84c2). Click here to learn what that means.
The diff coverage is n/a.

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@@          Coverage Diff           @@
##             2.6    #3491   +/-   ##
======================================
  Coverage       ?   66.45%           
======================================
  Files          ?     3205           
  Lines          ?    61567           
  Branches       ?     9497           
======================================
  Hits           ?    40913           
  Misses         ?    18383           
  Partials       ?     2271           

Flag	Coverage Δ
Linux	66.39% <0.00%> (?)
Windows	66.40% <0.00%> (?)

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[AMoo-Miki]

Looking at the past PRs for the same change, we have not included a changelog entry.