Update StackExchange.Redis version to 3.0.0

[mgravell]

Note

Medium Risk
A major-version bump of the core Redis client can affect connection, protocol, and command behavior across the library and tests; scope is limited to the version pin plus a targeted test connection option.

Overview
Bumps the central StackExchange.Redis package reference from 2.13.17 to 3.0.0 in Directory.Packages.props, so the whole solution picks up the 3.x client.

Test infrastructure is adjusted for 3.0 behavior: EndpointsFixture now sets AllowAdmin = true on cloned ConfigurationOptions when opening shared endpoint connections, with a note that Execute / ExecuteAsync handle admin commands more reliably in 3.0. That aligns fixture-created connections with tests that already rely on admin-style commands (e.g. FLUSHALL via Execute in AbstractNRedisStackTest).

Cosmetic newline-at-EOF fixes only in the touched files.

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[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}