Reduce number of ports that consul test agents take

[kisunji]

Description

After I increased test parallelism in #18999, I noticed random tests would panic due to not enough ports being free:

2023-10-03T16:41:26.3801137Z === �[31mFAIL�[0m: command/monitor TestMonitorCommand_exitsOnSignalBeforeLinesArrive (0.00s)
2023-10-03T16:41:26.3801752Z     testagent.go:417: failed to take 8 ports: %!w(*errors.errorString=&{freeport: block size too small})
2023-10-03T16:41:26.3802154Z panic: freeport: cannot allocate port block [recovered]
2023-10-03T16:41:26.3802542Z 	panic: freeport: cannot allocate port block [recovered]
2023-10-03T16:41:26.3802949Z 	panic: freeport: cannot allocate port block

This PR attempts to reduce reserved ports by 1, hoping to reduce the frequency of these panics.

Testing & Reproduction steps

Links

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

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2 Ignored Deployments

Name	Status	Preview	Updated (UTC)
consul	⬜️ Ignored (Inspect)		Oct 3, 2023 5:00pm
consul-ui-staging	⬜️ Ignored (Inspect)		Oct 3, 2023 5:00pm

[johnlanda]

Looks good to me!