Rename conan.cfg to global.conf

[czoido]

Changelog: Fix: Rename conan.cfg to global.conf.
Docs: conan-io/docs#2008

Closes: #8400

Also opened a PR to the release post: conan-io/conan-io.github.io#158

Naming of the new configuration file to conan.cfg was a bit confusing. Better than changing the block section [conf] this PR renames the file from conan.cfg to global.conf.

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