Disallow negative query boost

This change disallows negative query boosts. Negative scores are not allowed in Lucene 8 so
it is easier to just disallow negative boosts entirely. We should also deprecate negative boosts
in 6x in order to ensure that users are aware when they'll upgrade to ES 7.

Relates elastic#33309

docs/reference/migration/migrate_7_0/search.asciidoc

@@ -61,6 +61,7 @@ Scroll queries are not meant to be cached.
 
 [float]
 ==== Scroll queries cannot use `rescore`  anymore
+
 Including a rescore clause on a query that creates a scroll (`scroll=1m`) has
 been deprecated in 6.5 and will now return a `400 - Bad request`.  Allowing
 rescore on scroll queries would break the scroll sort.  In the 6.x line, the
@@ -123,3 +124,9 @@ max number of concurrent shard requests per node. The default is now `5`.
 
 `max_score` used to be set to `0` whenever scores are not tracked. `null` is now used
 instead which is a more appropriate value for a scenario where scores are not available.
+
+[float]
+==== Negative boosts are not allowed
+
+Setting a negative `boost` in a query, deprecated in 6x, are not allowed in this version.
+To deboost a specific query you can use a `boost` comprise between 0 and 1.

server/src/main/java/org/elasticsearch/index/query/AbstractQueryBuilder.java

@@ -159,6 +159,10 @@ public final float boost() {
     @SuppressWarnings("unchecked")
     @Override
     public final QB boost(float boost) {
+        if (Float.compare(boost, 0f) < 0) {
+            throw new IllegalArgumentException("negative [boost] are not allowed in [" + toString() + "], " +
+                "use a value between 0 and 1 to deboost");
+        }
         this.boost = boost;
         return (QB) this;
     }

test/framework/src/main/java/org/elasticsearch/test/AbstractQueryTestCase.java

@@ -101,6 +101,13 @@ public final QB createTestQueryBuilder() {
      */
     protected abstract QB doCreateTestQueryBuilder();
 
+    public void testNegativeBoosts() {
+        QB testQuery = createTestQueryBuilder();
+        IllegalArgumentException exc =
+            expectThrows(IllegalArgumentException.class, () -> testQuery.boost(-0.5f));
+        assertThat(exc.getMessage(), containsString("negative [boost]"));
+    }
+
     /**
      * Generic test that creates new query from the test query and checks both for equality
      * and asserts equality on the two queries.