Upgrade to otel 2.x and use core package instead

[dayaffe]

Issue #, if available:

Description of changes:

• Depend on opentelemetry-swift-core 2.3.0 instead of opentelemetry-swift 1.13
• Inline test-only InMemoryExporter (not vended by core package)
• Update README
• Fixes SPM target name conflict when both packages are in the dependency graph

Tested Locally

Test Suite 'OTelUtilsTests' passed at 2026-02-26 15:48:20.897.
         Executed 10 tests, with 0 failures (0 unexpected) in 0.002 (0.003) seconds
Test Suite 'smithy-swift-opentelemetryPackageTests.xctest' passed at 2026-02-26 15:48:20.897.
         Executed 19 tests, with 0 failures (0 unexpected) in 0.018 (0.021) seconds
Test Suite 'All tests' passed at 2026-02-26 15:48:20.897.
         Executed 19 tests, with 0 failures (0 unexpected) in 0.018 (0.025) seconds
◇ Test run started.
↳ Testing Library Version: 1501
↳ Target Platform: arm64e-apple-macos14.0
:heavy_check_mark: Test run with 0 tests in 0 suites passed after 0.001 seconds.

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