Follow POI 5.x repository directory structure

[9rnsr]

Current NPOI repository directory structure follows old POI (3.x?). It actually causes some difficulties during port from POI code.

I'm started working to fix issues, and I found some build process issues before change directory structures.

• [x] Scattering of npoi.snk file. (#1028)
• [x] Outdated packages.config files. (#1028)
• [x] Outdated build targets for older .NET Framework versions. (#1031)
• [x] Test cases are not runnable for net472 with VS2022. (#1032)
• [x] Inconsistent NuGet package versions between projects and target frameworks. (#1032)
• [x] Troublesome manual .runsettings file specification for testing. (#1037) It's not automatically detected so we cannot run test cases immediately after NPOI.Test.sln or NPOI.Test.Core.sln open.
• [x] Migrate csproj for net472 to SDK style. (#1037)
• [ ] Revive scratchpad projects.

My current plan of directory tree changes:

current	new	assembly name
/main	/npoi/main	NPOI.dll (keep)
/testcases/main	/npoi/test
/ooxml	/npoi-ooxml/main	NPOI.OOXML.dll (name is unchanged, but will contain additional classes from /openxml4Net)
/openxml4Net	/npoi-ooxml/main/openxml4Net	NPOI.OpenXml4Net.dll (removed)
/testcases/ooxml	/npoi-ooxml/test
/OpenXmlFormats	/OpenXmlFormats/main	NPOI.OpenXmlFormats.dll (keep) (or rename to NPOI.OOXML.Full.dll ?)
/testcases/openxml4net	/OpenXmlFormats/test
/scratchpad	/npoi-scratchpad/main	combine three csprojs into one
/testcases/scratchpad	/npoi-scratchpad/test
/testcases/test-data	/test-data