.Net: Remove planners - #12399

### Motivation and Context

The SK planners were deprecated and stopped being published as NuGet
packages at the end of April 2025 -
microsoft#11741.

This PR removes them from the codebase, leaving only the
StepwisePlannerMigration app to show how to migrate from SK planners to
auto function calling.

…ion (microsoft#14715)

### Motivation and Context

A full link-integrity scan of the repository's README files found 13
dead internal links in 4 files. The underlying sample files were deleted
or moved by recent refactors (planner removal microsoft#12399, MEVD Preview 2
microsoft#11028, MsGraph V5 update microsoft#12962, and the Gemini thinking-budget sample
rename in microsoft#13396), but the README entries still point at the old paths.

### Description

- **dotnet/samples/Concepts/README.md**
- `Google_GeminiChatCompletionWithThinkingBudget` -> renamed sample
`Google_GeminiChatCompletionWithThinking` (renamed in microsoft#13396)
- Removed the `VectorStore_DataIngestion_CustomMapper` entry (sample
deleted in microsoft#11028, no replacement)
- Removed the entire `Planners` section (all three samples deleted in
microsoft#12399)
- Removed `MsGraph_ContactsPlugin` / `MsGraph_DrivePlugin` /
`MsGraph_TasksPlugin` entries (no longer present after the MsGraph V5
update in microsoft#12962)
- Removed `WithFunctionCallingStepwisePlanner` entry (deleted together
with the planners in microsoft#12399)
- **dotnet/samples/Demos/OnnxSimpleRAG/README.md**
- Vector Store abstractions link ->
[Microsoft.Extensions.VectorData.Abstractions in
dotnet/extensions](https://github.com/dotnet/extensions/tree/main/src/Libraries/Microsoft.Extensions.VectorData.Abstractions)
(the `dotnet/src/VectorData/VectorData.Abstractions` README itself
points there)
- `InMemoryVectorStore` -> [CommunityToolkit/AI
MEVD/src/InMemory/InMemoryVectorStore.cs](https://github.com/CommunityToolkit/AI/blob/main/MEVD/src/InMemory/InMemoryVectorStore.cs)
(same pointer as `dotnet/src/VectorData/InMemory/README.md`)
- **python/semantic_kernel/connectors/memory_stores/redis/README.md**
- .NET README link -> `dotnet/src/VectorData/Redis/README.md` (current
in-repo location, which forwards to the CommunityToolkit repository)
- **python/samples/concepts/README.md**
- Search section link -> `python/semantic_kernel/connectors/search.py`
(the `connectors/search` directory no longer exists; search now lives in
`search.py`)

### Verification

All 5 new URLs were checked and return HTTP 200. All 13 removed/updated
links were verified to return HTTP 404 against `main` before this
change.

Docs-only change: no code is touched, so builds and unit tests are
unaffected.