Update CHANGELOG for 2.13.0-beta1

Remove build for .NET Core 1.x

Add NuGet.config back

Cherry-pick dotnet#835 and dotnet#818 for v2.12.1 release (dotnet#851)

* Allow rewriting of all malformed URIs regardless of target value (dotnet#835)

* Fixed `OpenXmlElement.RawOuterXml` to properly set null values without throwing (dotnet#818)

Co-authored-by: Alex Kindle <[email protected]>
Co-authored-by: aidilumarov <[email protected]>

Update README.md

Update CHANGELOG.md

Update changelog for 2.11.2 (dotnet#751)

Add date for 2.11.1 release (dotnet#748)

Update changelog for v2.11.0 (dotnet#728)

Add FileFormatVersions.Office2019 for completeness (dotnet#695)