remove quotes for otel auth config (elastic#186797)

### Summary

When doing configuration of an OpenTelemetry agent, most of the
environment variables are provided with only the value, but the auth
headers one is using quotes.

When using a shell script or YAML to set environment variables, the
value has to be quoted due to spaces, but it's not always the case, for
example when setting an environment variable in Windows.

Also, removing the quotes makes it consistent with other environment
variable values.

### Screenshot before

![Screenshot from 2024-06-24
11-21-52](https://github.com/elastic/kibana/assets/763082/8b2c513a-a5d2-4a50-b908-6d14240bf169)

x-pack/plugins/observability_solution/apm/public/tutorial/config_agent/opentelemetry_instructions.tsx

@@ -32,7 +32,7 @@ export function OpenTelemetryInstructions({ apmServerUrl, secretToken }: Props)
     },
     {
       setting: 'OTEL_EXPORTER_OTLP_HEADERS',
-      value: `"Authorization=Bearer ${secretToken ? secretToken : '<secret-token>'}"`,
+      value: `Authorization=Bearer ${secretToken ? secretToken : '<secret-token>'}`,
     },
     {
       setting: 'OTEL_METRICS_EXPORTER',