Release 1.7.0: define the next Cardano SL version as 3.1.0

[tatyanavych]

Description

Linked issue

Type of change

• [ ] 🐞 Bug fix (non-breaking change which fixes an issue)
• [ ] 🛠 New feature (non-breaking change which adds functionality)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)
• [ ] 🏭 Refactoring that does not change existing functionality but does improve things like code readability, structure etc
• [ ] 🔨 New or improved tests for existing code
• [ ] ⛑ git-flow chore (backport, hotfix, etc)

Developer checklist

• [ ] I have read the style guide document, and my code follows the code style of this project.
• [ ] If my code deals with exceptions, it follows the guidelines.
• [ ] I have updated any documentation accordingly, if needed. Documentation changes can be reflected in opening a PR on cardanodocs.com, amending the inline Haddock comments, any relevant README file or one of the document listed in the docs directory.
• [ ] CHANGELOG entry has been added and is linked to the correct PR on GitHub.

Testing checklist

• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

QA Steps

Screenshots (if available)

How to merge

Send the message bors r+ to merge this PR. For more information, see docs/how-to/bors.md.

[disassembler]

bors r+

[iohk-bors[bot]]

Build succeeded

• buildkite/cardano-sl
• ci/hydra:Cardano:cardano-sl:required