remove ubuntu18 from builds

[ericpassmore]

Summary

Removed ubuntu18 from builds, as we no longer support this version for CDT. Removing unbuntu18 allows us to proceed with additional upgrades and support later versions of c++, llvm, and clang/gcc.

Changes

in build.yaml removed unbuntu18 from list of builds and removed unbuntu18 specific compatibility steps.

Resolves #213

[spoonincode]

also remove

cdt/.cicd/platforms.json

Lines 1 to 4 in 0056647

	{
	"ubuntu18": {
	"dockerfile": ".cicd/platforms/ubuntu18.Dockerfile"
	},

and the file it is pointing to

[spoonincode]

also remove the .cicd/platforms/ubuntu18.Dockerfile file

[ericpassmore]

Note:start
group: RELEASE
category: CHORE
summary: Remove ubuntu-18 from CICD build and test.
Note:end