Fix problem in xray-lambda propagator definition

[tylerbenson]

the problem comes from this "late" addition:

To avoid potential issues when extracting with an active span context, the xray-lambda propagator SHOULD check if the provided context already has an active span context. If found, the propagator SHOULD return the provided context unmodified.

This is a problem because xray-lambda can't check the original context, only the context that is provided to it, which is modified by prior propagators in the chain.

This means if the original carrier has an x-ray context and an x-ray environment variable set, and if the xray propagator is configured as we expect it to be, then the xray-lambda will be either overwritten (if configured first), or will no-op because it detects an existing span context.

Proposed solution: change xray-lambda to replace xray in the configured list instead of work together with. This way, it is able to check the context before xray modifies it.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
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• schema-next.yaml updated with changes to existing conventions.