rename: kafka → samsa

Full project rename from kafka to samsa (after Gregor Samsa, who
woke one morning from uneasy dreams to find himself transformed).

- Module: github.com/metamorphosis-dev/kafka → samsa
- Binary: cmd/kafka/ → cmd/samsa/
- CSS: kafka.css → samsa.css
- UI: all 'kafka' product names, titles, localStorage keys → samsa
- localStorage keys: kafka-theme → samsa-theme, kafka-engines → samsa-engines
- OpenSearch: ShortName, LongName, description, URLs updated
- AGPL headers: 'kafka' → 'samsa'
- Docs, configs, examples updated
- Cache key prefix: kafka: → samsa:

internal/views/templates/results.html

@@ -7,7 +7,7 @@
         <circle cx="11" cy="11" r="8"/>
         <path d="m21 21-4.35-4.35"/>
       </svg>
-      <span>kafka</span>
+      <span>samsa</span>
     </a>
 
     <form class="header-search" method="GET" action="/search" role="search">