Add upgrading guide section for gradle-enterprise extension deprecation

[wolfs]

Adds an upgrading guide section for the deprecated gradle-enterprise extension. The section goes a little bit into detail why there isn't a develocity extension.

Rendered upgrading guide

[gitstream-cm]

🥷 Code experts: eskatos, jvandort

wolfs, lkasso have most 👩‍💻 activity in the files.
eskatos, wolfs have most 🧠 knowledge in the files.

See details

platforms/core-configuration/kotlin-dsl-integ-tests/src/integTest/kotlin/org/gradle/kotlin/dsl/integration/BuildScanIntegrationTest.kt

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platforms/core-configuration/kotlin-dsl/src/main/kotlin/org/gradle/kotlin/dsl/PluginDependenciesSpecExtensions.kt

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	wolfs	lkasso
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platforms/documentation/docs/src/docs/userguide/releases/upgrading/upgrading_version_8.adoc

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eskatos: 15%

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[alllex]

LGTM, with a small remark

[wolfs]

@lkasso Please take a look as well. Happy to include your improvements to the upgrading guide section in this or a follow-up PR.

[facewindu]

👍