[7.13](backport #25741) [Elastic Agent] Fix fleet-server.yml spec to not overwrite existing keys

[mergify]

This is an automatic backport of pull request #25741 done by Mergify.
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Changes to be committed:
	modified:   x-pack/elastic-agent/CHANGELOG.next.asciidoc
	modified:   x-pack/elastic-agent/pkg/agent/program/testdata/fleet_server-fleet-server.yml
	modified:   x-pack/elastic-agent/pkg/agent/program/testdata/fleet_server.yml
	modified:   x-pack/elastic-agent/pkg/agent/transpiler/rules.go
	modified:   x-pack/elastic-agent/pkg/agent/transpiler/rules_test.go
	modified:   x-pack/elastic-agent/spec/fleet-server.yml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   x-pack/elastic-agent/pkg/agent/program/supported.go

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[botelastic]

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[elasticmachine]

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• Build Cause: Pull request #25745 updated

• Start Time: 2021-05-18T04:27:52.256+0000

• Duration: 55 min 17 sec

• Commit: 9c575b4

Test stats 🧪

Test	Results
Failed	0
Passed	6860
Skipped	24
Total	6884

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Test stats 🧪

Test	Results
Failed	0
Passed	6860
Skipped	24
Total	6884

[blakerouse]

@Mergifyio rebase

[mergify]

Command rebase: success

Branch already up to date