docs: document systemcrypto experiment removal

[qmuntal]

Documents PR 2320 follow-up:

• removal of systemcrypto and nosystemcrypto GOEXPERIMENT user guidance
• MS_GO_NOSYSTEMCRYPTO opt-out guidance
• retained goexperiment build tags for source compatibility
• Go 1.27 Linux CGO_ENABLED=0 no-cgo OpenSSL behavior

Validation:

• git diff --cached --check

[Copilot]

Pull request overview

This PR updates the Microsoft build of Go documentation to reflect that systemcrypto/nosystemcrypto are no longer configured via GOEXPERIMENT starting in Go 1.27, and to document the MS_GO_NOSYSTEMCRYPTO=1 opt-out plus the Go 1.27 Linux CGO_ENABLED=0 (cgo-less OpenSSL) behavior.

Changes:

• Replaces/clarifies user guidance around GOEXPERIMENT=systemcrypto / GOEXPERIMENT=nosystemcrypto removal and the MS_GO_NOSYSTEMCRYPTO=1 opt-out.
• Updates no-cgo OpenSSL backend docs to describe Go 1.27+ automatic selection behavior (and Go 1.26’s experimental flag).
• Updates FIPS, migration, toolset-identification, and telemetry docs for consistency with the new configuration model.

Show a summary per file

File	Description
eng/doc/Telemetry.md	Clarifies what the msgo/systemcrypto:{enabled,disabled} telemetry counter represents (default selection + MS_GO_NOSYSTEMCRYPTO=1 opt-out).
eng/doc/NocgoOpenSSL.md	Updates the narrative to describe Go 1.27+ automatic cgo-less OpenSSL selection on Linux and notes the removed Go 1.26 experiment.
eng/doc/MigrationGuide.md	Updates migration guidance for Go 1.27+ removal of systemcrypto/nosystemcrypto from GOEXPERIMENT, and documents the no-cgo Linux behavior.
eng/doc/MicrosoftToolsetIdentification.md	Adjusts identification guidance to account for GOEXPERIMENT=systemcrypto/nosystemcrypto only being present in older binaries.
eng/doc/fips/README.md	Reframes build configuration guidance around default selection + MS_GO_NOSYSTEMCRYPTO=1, retaining legacy build-tag compatibility notes.
eng/doc/AdditionalFeatures.md	Updates the high-level feature overview to match Go 1.27’s configuration approach and opt-out mechanism.

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 1