Update another set of PropertyStore usages

[JeremyKuhne]

This actually removes one of the legacy methods, so yay progress.

In Form the owned forms set is now a List<Form> rather than a manually managed array.

Microsoft Reviewers: Open in CodeFlow

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.50519% with 39 lines in your changes missing coverage. Please review.

Project coverage is 75.38275%. Comparing base (79def1d) to head (f2dade7). Report is 4 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #12209 +/- ## =================================================== - Coverage 75.38975% 75.38275% -0.00701% =================================================== Files 3100 3100 Lines 634382 634165 -217 Branches 46866 46847 -19 =================================================== - Hits 478259 478051 -208 + Misses 152714 152691 -23 - Partials 3409 3423 +14 ``` | [Flag](https://app.codecov.io/gh/dotnet/winforms/pull/12209/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/12209/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `75.38275% <86.50519%> (-0.00701%)` | :arrow_down: | | [integration](https://app.codecov.io/gh/dotnet/winforms/pull/12209/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `17.98510% <47.91667%> (-0.01628%)` | :arrow_down: | | [production](https://app.codecov.io/gh/dotnet/winforms/pull/12209/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `48.72542% <86.45833%> (-0.03579%)` | :arrow_down: | | [test](https://app.codecov.io/gh/dotnet/winforms/pull/12209/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `97.02680% <100.00000%> (-0.00002%)` | :arrow_down: | | [unit](https://app.codecov.io/gh/dotnet/winforms/pull/12209/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dotnet) | `45.76031% <86.45833%> (-0.03453%)` | :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.