netflow: add missing read buffer size configuration

[efd6]

What does this PR do?

Adds a missing read buffer size configuration.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Fixes [Netflow] Missing read_buffer setting in UI configuration  #5682

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Build stats

• Start Time: 2023-03-28T01:57:06.928+0000

• Duration: 15 min 52 sec

Test stats 🧪

Test	Results
Failed	0
Passed	14
Skipped	0
Total	14

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (1/1)	💚
Classes	100.0% (1/1)	💚
Methods	100.0% (8/8)	💚 8.621
Lines	90.909% (80/88)	👎 -8.439
Conditionals	100.0% (0/0)	💚

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Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

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Package netflow - 2.6.1 containing this change is available at https://epr.elastic.co/search?package=netflow