Fix serde migration function

[JakeSCahill]

Partially resolves redpanda-data/console#1721

This pull request includes several changes to the console-config-migrator/main.go file, focusing on improving the migration of Kafka and console settings and updating the authentication migration. The most important changes are listed below:

Kafka and Console Settings Migration:

• Modified migrateKafka function to return serde settings along with Kafka settings and warnings. This change ensures that serde settings from Kafka are correctly merged with console serde settings. [1] [2]
• Updated Convert function to merge serde settings from both Kafka and console migrations into a single serde block. This ensures that all serde settings are consolidated and correctly applied.

Authentication Migration:

• Changed the key name from jwtSigningSecret to jwtSigningKey in the migrateAuthentication function.

These changes improve the overall configuration migration process by ensuring that settings are correctly merged and correctly named.

Test case

Paste the following into the migration tool:

kafka:
  broker: "localhost:9092"
  schemaRegistry:
    username: "sruser"
    password: "srpass"
    bearerToken: "srbearer"
    url: "http://schema-registry:8081"
  protobuf:
    enabled: true
  cbor:
    enabled: false
  messagePack:
    enabled: true

You should see:

kafka:
  broker: "localhost:9092"
  sasl:
    enabled: true
    impersonateUser: true
schemaRegistry:
  authentication:
    basic:
      username: sruser
      password: srpass
    bearerToken: srbearer
    impersonateUser: false
  url: "http://schema-registry:8081"
serde:
  protobuf:
    enabled: true
  cbor:
    enabled: false
  messagePack:
    enabled: true

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[asimms41]

LGTM. Migration tool works as described.