Remove docs reference to ephemeral encryption key

[legrego]

We do not generate an ephemeral encryption key anymore. This was removed in #81511, so we should update our docs to reflect our new reality.

kibana/docs/user/security/secure-saved-objects.asciidoc

Lines 17 to 20 in d4530e1

	[IMPORTANT]
	============================================================================
	If you don't specify an encryption key, {kib} automatically generates a random key at startup. Every time you restart {kib}, it uses a new ephemeral encryption key and is unable to decrypt saved objects encrypted with another key. To prevent data loss, {kib} might disable features that rely on this encryption until you explicitly set an encryption key.
	============================================================================

Originally posted by @legrego in #129131 (comment)

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)