docs: update course_unit_sidebar slot ID to v2 in plugin-slots README

[brian-smith-tcril]

The CourseAuthoringUnitSidebarSlot README defines the slot ID as org.openedx.frontend.authoring.course_unit_sidebar.v2, but the top-level plugin-slots README was referencing it as v1. This fixes that inconsistency.

🤖 Generated with Claude Code

[brian-smith-tcril]

Context: I was discussing plugin slots and mentioned having a slot we've made a v2 version of. I searched the org for it (by searching the readmes in the plugin-slots dirs) but couldn't find a v2. Asked claude to find it, once it found it I asked it to make a PR updating the readme.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.37%. Comparing base (5672644) to head (2be67cd).

Additional details and impacted files

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##           master    #2905   +/-   ##
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  Coverage   95.37%   95.37%           
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  Files        1332     1332           
  Lines       30594    30594           
  Branches     6925     6919    -6     
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  Hits        29178    29178           
+ Misses       1359     1347   -12     
- Partials       57       69   +12     

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[bradenmacdonald]

We currently have both a v1 slot (on the legacy sidebar) and a v2 slot (on the new sidebar) so I think the README should mention both. CC @ChrisChV to double check.

[brian-smith-tcril]

We currently have both a v1 slot (on the legacy sidebar) and a v2 slot (on the new sidebar) so I think the README should mention both. CC @ChrisChV to double check.

I was thinking since we recommend against using v1 (from the v1 README):

This document describes an older version v1 of the CourseAuthoringUnitSidebarSlot. It is recommended to use the org.openedx.frontend.authoring.course_unit_sidebar.v2 slot ID for new plugins.

The v1 slot has the following limitations compared to the v2 version:

• It renders conditionally based on the isUnitVerticalType prop, which means the plugins won't be rendered in other scenarios like unit with library blocks.
• It does not wrap the SplitTestSidebarInfo component. So it can't be hidden from the sidebar by overriding the components in the slot.
• As it is not the primary child component of the sidebar, CSS styling for inserted components face limitations, such as an inability to be sticky or achieve 100% height.

and we mention v1 on the v2 page, just having v2 in the README better communicates "you should use v2." v1 purely exists as a means to not break plugins made before the v2 improvements.

[ChrisChV]

Before the new sidebar changes, v2 already existed in that position, and v1 was used within it (ref). With the new changes, v1 has become the legacy sidebar and v2 the new sidebar. In the near future, there are plans to deprecate and remove that legacy code (v1)

[bradenmacdonald]

OK, sounds good!