groovy to hirsute on arm64: KeyError: 'ubuntu-desktop-rapsi'

I did a release-upgrade from groovy to hirsute on my groovy pi4, and got this backtrace:

(...)
KeyError: "The cache has no package named 'ubuntu-desktop-rapsi'"
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/problem_report.py", line 477, in add_to_existing
    self.write(f)
  File "/usr/lib/python3/dist-packages/problem_report.py", line 430, in write
    block = f.read(1048576)
  File "/usr/lib/python3.8/codecs.py", line 322, in decode
    (result, consumed) = self._buffer_decode(data, self.errors, final)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x8b in position 1: invalid start byte

Original exception was:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 281, in __getitem__
    rawpkg = self._cache[key]
KeyError: 'ubuntu-desktop-rapsi'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/tmp/ubuntu-release-upgrader-42aqo0hw/hirsute", line 8, in <module>
    sys.exit(main())
  File "/tmp/ubuntu-release-upgrader-42aqo0hw/DistUpgrade/DistUpgradeMain.py", line 236, in main
    if app.run():
  File "/tmp/ubuntu-release-upgrader-42aqo0hw/DistUpgrade/DistUpgradeController.py", line 2024, in run
    return self.fullUpgrade()
  File "/tmp/ubuntu-release-upgrader-42aqo0hw/DistUpgrade/DistUpgradeController.py", line 2005, in fullUpgrade
    self.doPostUpgrade()
  File "/tmp/ubuntu-release-upgrader-42aqo0hw/DistUpgrade/DistUpgradeController.py", line 1395, in doPostUpgrade
    self.quirks.run("PostUpgrade")
  File "/tmp/ubuntu-release-upgrader-42aqo0hw/DistUpgrade/DistUpgradeQuirks.py", line 99, in run
    func()
  File "/tmp/ubuntu-release-upgrader-42aqo0hw/DistUpgrade/DistUpgradeQuirks.py", line 146, in hirsutePostUpgrade
    if cache['ubuntu-desktop-rapsi'].is_installed:
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 283, in __getitem__
    raise KeyError('The cache has no package named %r' % key)
KeyError: "The cache has no package named 'ubuntu-desktop-rapsi'"