chore: Correct file/test data source and web proxy feature metadata

[kinyoklion]

Summary

• Marks fileDataSource and testDataSource as removed in 3.0 for the C++ server SDK. Both are v2.x C server SDK features; the 3.x SDK has no file or test data source, but the feature matrix implied current support (introduced 2.6 with no removal).
• Records webProxy as introduced in 3.11 for the C++ client SDK (HttpPropertiesBuilder::Proxy / LDClientConfigBuilder_HttpProperties_Proxy, available when the SDK is built with -DLD_CURL_NETWORKING=ON).

Validated by running sdk-meta's ingest tool against the edited file and confirming the resulting feature rows.

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Note

Overview
Updates .sdk_metadata.json so the C++ SDK feature matrix matches actual product support.

For the C++ server SDK, fileDataSource and testDataSource now include removed": "3.0" alongside their 2.6 introduction. That stops the matrix from implying current support in 3.x, where those v2.x C server data-source features are not present.

For the C++ client SDK, webProxy is added with introduced": "3.11", reflecting configurable HTTP proxy support (e.g. HttpPropertiesBuilder::Proxy / LDClientConfigBuilder_HttpProperties_Proxy when built with CURL networking).

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