[Automation] Bump Golang version to 1.21.6

[apmmachine]

It requires the version to be bumped first in golang-crossbuild project, then a new release will be added to:
https://github.com/elastic/golang-crossbuild/releases/tag/v1.21.6.
Otherwise it will fail until the docker images are available.

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Bump golang-version to latest version

Update .go-version

1 file(s) updated with "1.21.6": * .go-version

1.21.6

no GitHub Release found for go1.21.6 on "https://github.com/golang/go"

Update .golangci.yml

1 file(s) updated with "1.21.6": * .golangci.yml

1.21.6

no GitHub Release found for go1.21.6 on "https://github.com/golang/go"

Update version.asciidoc

1 file(s) updated with ":go-version: 1.21.6": * version/docs/version.asciidoc

1.21.6

no GitHub Release found for go1.21.6 on "https://github.com/golang/go"

Update from dockerfiles

2 file(s) updated with "ARG GO_VERSION=1.21.6": * Dockerfile * Dockerfile.skaffold

1.21.6

no GitHub Release found for go1.21.6 on "https://github.com/golang/go"

GitHub Action workflow link

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[elastic-sonarqube]

Quality Gate passed

Kudos, no new issues were introduced!

0 New issues
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube

[cmacknz]

=== RUN   TestDiagnosticsCommand
    fixture.go:261: Extracting artifact elastic-agent-8.13.0-SNAPSHOT-windows-x86_64.zip to C:\Users\windows\AppData\Local\Temp\TestDiagnosticsCommand1043038821\001
    fixture.go:274: Completed extraction of artifact elastic-agent-8.13.0-SNAPSHOT-windows-x86_64.zip to C:\Users\windows\AppData\Local\Temp\TestDiagnosticsCommand1043038821\001
    fixture.go:877: All component specifications where removed
    fixture.go:918: Placed component specifications: fake, fake-shipper
    diagnostics_test.go:137: 
        	Error Trace:	C:/Users/windows/agent/testing/integration/diagnostics_test.go:137
        	Error:      	Received unexpected error:
        	            	elastic-agent client received unexpected error: rpc error: code = Unavailable desc = connection error: desc = "transport: Error while dialing: open \\\\.\\pipe\\IS0qswTSLNZkV-zZ-Z3lrqmBo8RFmedV.sock: The system cannot find the file specified."
        	Test:       	TestDiagnosticsCommand
--- FAIL: TestDiagnosticsCommand (33.80s)

Hmm, this failure has appeared across a few tests now I think.

[cmacknz]

#4076

Failure is unrelated, force merging.