Update 'Managing LexoRank' documentation

Issue Summary

The document Managing LexoRank is outdated, as it still mentions 3 separate thresholds, but since Jira 8.9.0 there are only 2:

• 128 characters: a rebalance is scheduled for the next 12 hours.
• 160 characters: an immediate rebalance starts.

Also, Jira no longer stops all ranking operations when rank length reaches 254. Instead, ranking operations that would yield values having a length larger than 254 characters will fail, without affecting other ranking operations that do not meet this criterion.

This is reproducible on Data Center: yes

Steps to Reproduce

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Expected Results

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Actual Results

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Workaround

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