Update script to draft release notes

[remi-kazeroni]

Description

This PR:

1. updates the dates of the previous release to v2.7
2. adds a base=main argument to only list PRs that were merged into main. For example, for the Core, we would otherwise get PRs like: [v2.7.1 bugfix release] remove unnecessary test in regrid to allow install of iris 3.4 - feature from PR 1846 ESMValCore#1857 not merged into main

• Closes Drafting the changelog with PRs merged into the main branch only #2882
• Link to documentation:

---

Before you get started

• ☝ Create an issue to discuss what you are going to do

Checklist

It is the responsibility of the author to make sure the pull request is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• 🛠 This pull request has a descriptive title
• 🛠 Code is written according to the code quality guidelines
• 🛠 Documentation is available
• 🛠 Tests run successfully
  - [ ] 🛠 The list of authors is up to date
  - [ ] 🛠 Any changed dependencies have been added or removed correctly
• 🛠 All checks below this pull request were successful

---

To help with the number of pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository

[valeriupredoi]

thanks @remi-kazeroni - and cheers for cleaning up what clearly is one of my things I forgot to change (had run that script manually for 2.7)

[valeriupredoi]

cheers Remi and Saskia, merging this!