Update Linux Build Instructions

[jscottbranson]

High Level Overview of Change

The rippled Linux build instructions have conflicting information - in some places they suggest repo versions of protobuf are sufficient while other lines explicitly mandate that v2 is required. Debian 10+ as well as Ubuntu 18.04+ ship with protobuf v3.

I also removed a reference to Ubuntu 16.04, since it seems like many of the dependence (including protobuf and the compiler) would be out of date, so it would be far easier to build on 18.04. However, it might make sense to leave the reference in, since 16.04 is not EoL.

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [ ] Tests (You added tests for code that already exists, or your new feature included in this PR)
• [x] Documentation Updates
• [ ] Release

[jscottbranson]

Thanks for the additional edits, @legleux. I've updated the build instructions, so they now automatically detect the number of processor cores. I also changed the my_build directory to build, so it is picked up by .gitignore.

[nbougalis]

Hey @crypticrabbit, can you please squash this commit down to one or two, as appropriate, and rebase it to the latest develop? Thanks.

[intelliot]

(notes: will squash this to 1 commit)