Fix the document: Task Level Comp Res is beta

[khrm]

Task-level Resource Requirements are beta. taskruns.md says it is alpha. https://github.com/tektoncd/pipeline/blob/main/docs/additional-configs.md#beta-features correctly specify as beta.

Fixes #8005

Changes

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Release Notes

NONE

[khrm]

/kind documentation

[khrm]

/assign @chitrangpatel @vdemeester

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JeromeJu, vdemeester

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[JeromeJu]

/lgtm