Remove remaining old PropertyStore APIs

[JeremyKuhne]

• Tweak AddOrRemove methods for better change comparison
• Make it consistently throw if trying to force cast null to T

Follow up:

• Try and lock down harder on insertion of null values. Considering using a sentinel perhaps.
• Try to move away from CreateKey. Considering using enum(s), making PropertyStore generic, etc.

  Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 74.55830% with 72 lines in your changes missing coverage. Please review.

Project coverage is 75.41122%. Comparing base (af25023) to head (86ec3d9). Report is 10 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #12215 +/- ## =================================================== + Coverage 75.40907% 75.41122% +0.00215% =================================================== Files 3101 3101 Lines 634307 634151 -156 Branches 46867 46850 -17 =================================================== - Hits 478325 478221 -104 + Misses 152567 152506 -61 - Partials 3415 3424 +9 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/12215/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | Coverage Δ | | |---|---|---| | [Debug](https://app.codecov.io/gh/dotnet/winforms/pull/12215/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `75.41122% <74.55830%> (+0.00215%)` | :arrow_up: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/12215/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.99145% <31.57895%> (+0.00105%)` | :arrow_up: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/12215/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `48.77285% <57.89474%> (+0.00027%)` | :arrow_up: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/12215/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `97.02696% <100.00000%> (-0.00047%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/12215/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `45.80036% <56.14035%> (-0.00466%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet#carryforward-flags-in-the-pull-request-comment) to find out more.

[JeremyKuhne]

I tried using enums for keys. It can be done, it is just awkward. I'm going to try using a source generator as an alternative. With a source generator I could keep visibility identical and still turn the key values into constants while avoiding collisions.