cli/config/configfile: deprecate ConfigFile.Experimental field

[thaJeztah]

relates to:

• Always enable experimental features #2774
• Deprecate configuration options for experimental CLI features #2773
• [19.03 backport] Deprecate configuration options for experimental CLI features #2776

Configuration options for experimental CLI features were deprecated in docker 19.03 (3172219), and enabled by default since docker 20.10 (977d3ae).

This deprecates the corresponding field in the config-file.

- Human readable description for the release notes

Go SDK: cli/config/configfile: deprecate `ConfigFile.Experimental` field. Experimental CLI features are always enabled since version v20.10 and this field is no longer used. Use `ConfigFile.Features` instead for optional features. This field will be removed in a future release.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.12%. Comparing base (3349492) to head (c8f9187).

Additional details and impacted files

@@ Coverage Diff @@## master #5977 +/- ##
=======================================
Coverage 59.12% 59.12% =======================================
Files 355 355 Lines 29740 29740 =======================================
Hits 17583 17583 Misses 11182 11182 Partials 975 975 

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[thaJeztah]

Booh! I was afraid it would complain; I tried to keep the diff minimal, but I may have to break it up into multiple lines;

#15 68.00 cli/config/configfile/file.go:39: line-length-limit: line is 273 characters, out of limit 200 (revive)
#15 68.00 Experimental string `json:"experimental,omitempty"` // Deprecated: experimental CLI features are always enabled and this field is no longer used. Use [Features] instead for optional 

[thaJeztah]

@vvoland@Benehiko ptal 🤗