Remove Ubuntu-18.04 jobs since github deprecated them

[TGWDB]

This is in response to this announcement.

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[thomasspriggs]

Looks good so far, we should remove the references to Ubuntu 18 from the work flows documentation -

cbmc/.github/workflows/README.md

Line 41 in 888a08a

* an `ubuntu-18.04` package

and from the release page text -

cbmc/.github/workflows/regular-release.yaml

Line 62 in 888a08a

# Ubuntu 18:

I think the release text also needs Ubuntu 22.04 adding to it, as I think this was missed when we added the 22.04 package to the releases

[TGWDB]

Looks good so far, we should remove the references to Ubuntu 18 from the work flows documentation -

cbmc/.github/workflows/README.md

Line 41 in 888a08a

* an `ubuntu-18.04` package

and from the release page text -

cbmc/.github/workflows/regular-release.yaml

Line 62 in 888a08a

# Ubuntu 18:

I think the release text also needs Ubuntu 22.04 adding to it, as I think this was missed when we added the 22.04 package to the releases

Fixed both, thanks.

[NlightNFotis]

This change overshot slightly - we shouldn't delete the headers or the instruction text, just the dpkg instructions for Ubuntu 18.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (888a08a) 78.51% compared to head (37b4bd7) 78.51%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7657   +/-   ##
========================================
  Coverage    78.51%   78.51%           
========================================
  Files         1674     1674           
  Lines       191935   191935           
========================================
  Hits        150704   150704           
  Misses       41231    41231           

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