[Fleet] Update max spec version to 3.6

[jsoriano]

Summary

Kibana 9.4.0 is intended to support Package Spec 3.6. Update default maximum value and serverless configuration accordingly.

For related issues, see elastic/package-spec#1060.

Checklist

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Reviewers should verify this PR satisfies this list as well.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines
• Review the backport guidelines and apply applicable backport:* labels.

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[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[macroscopeapp]

Approvability

Verdict: Needs human review

Simple version bump (3.5 → 3.6) for the Fleet registry max spec version, but the author does not own either changed file. The designated code owners (@elastic/fleet, @elastic/kibana-core, @elastic/kibana-security) should review this configuration change.

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