[DOCS] Fix impact sentences for removed system properties (#77601)

Several sentences in the 8.0 breaking changes reference setting
system properties in `elasticsearch.yml`, which is not supported. This corrects
those sentences.

It also fixes a sentence that references the `http.content_type.required`
setting as a system property.

docs/reference/migration/migrate_8_0/allocation.asciidoc

@@ -21,8 +21,7 @@ longer optional, and this system property must now not be set.
 
 *Impact* +
 Discontinue use of the `es.disk.auto_release_flood_stage_block` system property.
-Specifying this property in `elasticsearch.yml` will result in an error on
-startup.
+Setting this system property will result in an error on startup.
 ====
 
 [[breaking_80_allocation_change_include_relocations_removed]]

docs/reference/migration/migrate_8_0/http.asciidoc

@@ -16,9 +16,8 @@ Elasticsearch 5.3 to prepare users for Elasticsearch 6.0, where content type
 auto detection was removed for HTTP requests.
 
 *Impact* +
-Discontinue use of the `http.content_type.required` system property.
-Specifying this property in `elasticsearch.yml` will result in an error on
-startup.
+Discontinue use of the `http.content_type.required` setting. Specifying this
+setting in `elasticsearch.yml` will result in an error on startup.
 ====
 
 .The `http.tcp_no_delay` setting has been replaced by `http.tcp.no_delay`.
@@ -44,9 +43,6 @@ In these previous versions, if your application required handling `+` as a singl
 to be supported in version 8.
 
 *Impact* +
-Update your workflow and applications to assume `+` in a URL is encoded as
-`%2B`. Discontinue use of the `es.rest.url_plus_as_space` system property.
-Specifying this property in `elasticsearch.yml` will result in an error on
-startup.
+Update your application or workflow to assume `+` in a URL is encoded as `%2B`.
 ====
 // end::notable-breaking-changes[]