Add transaction.name mapping to logs-apm.error

[marclop]

Motivation/summary

Adds the missing mapping for transaction.name to logs-apm.error.

Checklist

- [ ] Update CHANGELOG.asciidoc
- [ ] Update package changelog.yml (only if changes to apmpackage have been made)
- [ ] Documentation has been updated

How to test these changes

1. docker-compose up -d
2. cd systemtest && go test -v -run TestErrorIngest .
3. Navigate to Kibana > Dev tools

GET logs-apm.error*/_search
{
  "query": {
    "exists": {
      "field": "transaction.name"
    }
  }, 
  "_source": [false], 
  "fields": [
    "transaction.type",
    "transaction.name"
  ]
}

Verify that the query returns a result with the transaction.name set.

Related issues

Fixes regression found in #6539

[ghost]

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Build stats

• Start Time: 2021-12-22T08:43:44.190+0000

• Duration: 41 min 56 sec

• Commit: 29a4359

Test stats 🧪

Test	Results
Failed	0
Passed	5617
Skipped	19
Total	5636

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[simitt]

Please release a new apmpackage to staging when merging this.