[8.15] Add missing environment variables for container setup (backport #1354)

[mergify]

This updates the Elastic Agent environment variables page to add/remove variables as described here.

For easier reviewing, I've copied the output into this google doc.

Removed variables:

• KIBANA_FLEET_SETUP has been removed for a while: Remove KibanaSetup call as it is not needed anymore elastic-agent#2910

Added variables:

• FLEET_FORCE - same as the --force flag
• ELASTIC_AGENT_CERT & ELASTIC_AGENT_CERT_KEY - paths to mtls cert and key the agent will use to connect to fleet-server. (corresponding var for passphrase is missing)
• FLEET_SERVER_CLIENT_AUTH - set fleet-server mtls settings, one of: none (default), optional, required
• FLEET_SERVER_ES_CERT & FLEET_SERVER_ES_CERT_KEY - fleet-server -> es mTLS cert paths. (corresponding var for passphrase is missing)
• FLEET_SERVER_INSECURE_HTTP - expose fleet-server as an http server
• FLEET_HEADER - I think this one is headers the agent will send to fleet-server (internal setting according to Craig)
• FLEET_KIBANA_HEADER - headers used when contacting Kibana (internal setting according to Craig)
• FLEET_SERVER_TIMEOUT - how long elastic-agent will wait for fleet-server to checkin as healthy
• FLEET_DAEMON_TIMEOUT - how long fleet-server will wait during the bootstrap process for elastic-agent

Closes: #1355
Rel: #1075

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This is an automatic backport of pull request #1354 done by Mergify.

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