Fix PublicAPI build checks

[corranrogue9]

Currently, we don't get build errors when the public API surface has changed but the corresponding PublicAPI.Unshipped.txt has not been updated. This is because we get warnings that there are duplicate entries in PublicAPI.Unshipped.txt and PublicAPI.Shipped.txt. Because the analyzer doesn't know what to do when it encounters those warnings, it doesn't go on to generate the errors that there are missing entries in the text file (effectively disabling the public API build checks).

This change upgrades those duplicate entry warnings to errors, and it also upgrades the warning that an entry exists in the text file but doesn't correspond to a public API. These two will prevent us from accidentally "disabling" the public API build checks in the future.

This change also addresses the issues in the existing PublicAPI.*.txt files, removing the duplicate entries from the Unshipped variants.

It also adds support in Core and Client for multiple target frameworks in preparation for #2433.

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This PR has 105 quantified lines of changes. In general, a change size of upto 200 lines is ideal for the best PR experience!

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Quantification details

Label      : Medium
Size       : +73 -32
Percentile : 41%

Total files changed: 12

Change summary by file extension:
.csproj : +22 -5
.txt : +51 -27

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[chrisspre]

Does this not belong in the ../Build.props?

[corranrogue9]

We use Build.props in our test projects as well, which we don't ship publicly. I can try to consolidate the csproj elements used by our shipped projects into a .props file though if you think that makes sense.

[mikepizzo]