[docs-patrol] Update Kafka SASL mechanism docs for OAUTHBEARER support

Recent code changes introduced documentation drift for Kafka SASL configuration.

## Changes Requiring Documentation Updates

### 1. Kafka `sasl.mechanism` now supports `OAUTHBEARER`, but docs still list only three mechanisms

**Commit(s):** `d4a9f2871a2fa20a5c6d55028cdee1d5c5765278`

**What changed (code evidence):**
- `libbeat/common/kafka/sasl.go:34` adds `saslTypeOauthBearer = sarama.SASLTypeOAuth`.
- `libbeat/common/kafka/sasl_nofips.go:31` allows `saslTypeOauthBearer` in validation (`case "", saslTypePlaintext, saslTypeSCRAMSHA256, saslTypeSCRAMSHA512, saslTypeOauthBearer:`).

**Documentation impact (current drift evidence):**
- `docs/reference/filebeat/kafka-output.md:99-103` still says SASL mechanism can be one of `PLAIN`, `SCRAM-SHA-256`, `SCRAM-SHA-512`.
- `docs/reference/filebeat/filebeat-input-kafka.md:216-220` lists the same three values only.
- `docs/reference/metricbeat/metricbeat-module-kafka.md:99` says `Can be one of PLAIN, SCRAM-SHA-256 or SCRAM-SHA-512.`
- The same 3-value list also remains in other beat Kafka output docs (`auditbeat`, `heartbeat`, `metricbeat`, `packetbeat`, `winlogbeat`).

A user following the current docs will not discover a supported SASL mechanism.

## Suggested Actions

- [ ] Update all Kafka output/input/module docs that enumerate `sasl.mechanism` values to include `OAUTHBEARER`.
- [ ] Ensure shared Kafka docs snippets/templates (if any) include the new mechanism so generated docs stay consistent across beats.
- [ ] Optionally align related validation/error-message wording with the expanded mechanism list where applicable.

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Start with the Kafka documentation files named in the issue, including docs/reference/filebeat/kafka-output.md, filebeat-input-kafka.md, and metricbeat-module-kafka.md, then search the other Beat Kafka docs for the same three-value wording. Check the Kafka SASL validation files and any shared documentation snippets before editing. Done means all relevant enumerations consistently include OAUTHBEARER and the docs remain aligned with the supported configuration.

Written by the indexing model from the issue text.