Iris 3.2 release tasks

[lbdreyer]

Complete tasks set out in the "Release" page in the Iris docs.

Release Candidate

• Cut the v3.2.x branch from main
• Check iris-grib works with the v3.2.x branch
• Finalise the whats new + Update version string to 3.2.0rc0 3.2 version and whats new #4559
• Tag "v3.2.0rc0" pre release
• Check the readthedocs build (may need to activate this)
• Update conda-forge iris-feedstock using rc_iris label
• Prepare tweet announcing release candidate Add logo for 3.2.0rc0 twitter-scitools-iris#1
  • Also see https://github.com/SciTools/twitter-scitools-iris, to bank any code et al associated with tweet material
• Reset whatsnew Reset whats new #4563
• Update version on main branch Update version to 3.3.dev0 #4565

Release

• Update version string to 3.2.0 and whats new with release date Finalise whatsnew and version string update #4588
• Tag "v3.2.0"
• Update conda-forge iris-feedstock Update to v3.2.0 conda-forge/iris-feedstock#79
• Update PyPI
• Prepare tweet announcing release Add code and image for Iris 3.2 twitter-scitools-iris#2

Post Release

• Merge back v3.2.x branch onto main after the release Mergeback v3.2.x branch into main #4591
• Update release docs to include any missing steps