[APM] Trace waterfall: Change the service legend to use the EuiChart legend styles

[formgeist]

Summary

Related to #90619 (which might make this issue redundant)

Currently this is using a custom legend style which is similar but not consistent (sizing is different). Either we convert it to use the EuiChart legend component or alternatively, change it to use EuiHealth with custom visualization colors. If there's a simple replacement strategy vs. waiting until we replace the full waterfall visualization in #90619

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[botelastic]

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