Updating focal to groovy fails

[Mario Camou]

Running `do-release-upgrade` from focal to groovy, I got the following crash after the packages were downloaded:

```
apt_pkg.Error: E:Could not configure 'libc6:amd64'. , E:Could not perform immediate configuration on 'libnss-nis:amd64'. Please see man 5 apt.conf under APT::Immediate-Configure for details. (2)
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:
apt_pkg.Error: E:Could not configure 'libc6:amd64'. , E:Could not perform immediate configuration on 'libnss-nis:amd64'. Please see man 5 apt.conf under APT::Immediate-Configure for details. (2)
Error in function: install

apt_pkg.Error: E:Could not perform immediate configuration on 'libffi8ubuntu1:amd64'. Please see man 5 apt.conf under APT::Immediate-Configure for details. (2)
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:
apt_pkg.Error: E:Could not perform immediate configuration on 'libffi8ubuntu1:amd64'. Please see man 5 apt.conf under APT::Immediate-Configure for details. (2)

Could not install the upgrades

The upgrade has aborted. Your system could be in an unusable state. A
recovery will run now (dpkg --configure -a).

Upgrade infeasible

The upgrade could not be completed, there were errors during the
upgrade process.
```

I tried adding an /etc/apt/99temp-release-upgrade file with the following contents:

```
APT::Immediate-Configure "false"
```

which resulted in:

```
# do-release-upgrade
Traceback (most recent call last):
  File "/usr/bin/do-release-upgrade", line 11, in <module>
    from UpdateManager.Core.MetaRelease import MetaReleaseCore
  File "/usr/lib/python3/dist-packages/UpdateManager/Core/MetaRelease.py", line 25, in <module>
    import apt
  File "/usr/lib/python3/dist-packages/apt/__init__.py", line 35, in <module>
    apt_pkg.init_config()
apt_pkg.Error: E:Syntax error /etc/apt/apt.conf.d/99temp-for-release-upgrade:3: Extra junk at end of file
```

This was detected by someone during the beta, but it seems that no one took the time to fix it: https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1899272

[Mario Camou (mcamou)]

In case other people have this problem, I have been able to work around it by using the procedure given in the answer to this StackOverflow post: https://askubuntu.com/questions/458201/how-can-i-upgrade-ubuntu-to-newer-release-manually-without-using-update-manager