Reflect Tracing moving to Core in data/features.yml

Why is this change being made?

According to https://docs.gitlab.com/ee/operations/tracing.html we've moved in to Core in 13.5 and announced it 13.6.

This MR reflects this fact in data/features.yml to correct https://about.gitlab.com/features/.

This was a question by @awaters3 in Slack (internal):

hi - is Tracing part of Core; on the Features page it says Ultimate, but the linked doc says all tiers?

Screenshots

https://pl-features-tracing-core.about.gitlab-review.app/features/

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