[META] Release 1.0.0

[stevejgordon]

We will use this issue to track our plans for stabilising Elastic.Transport for a 1.0 release. There are several refactorings and enhancements to the design we would like to focus on. After reviewing the existing code, architecture and future requirements, we will flesh out this issue with new tasks and sub-issues.

Release 1.0.0 work

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1. Simplify TargetFrameworks and remove unsupported runtimes #70
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2. Prefer Microsoft.IO.RecyclableMemoryStream via NuGet package #71
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3. Update all code with nullable annotations and attributes #72
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4. Ensure all public members and types have full XML documentation comments #73
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5. Port more tests from the v7 NEST codebase #74
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6. Update and enhance OTEL instrumentation #75
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7. Rework RequestPipeline design #40
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