Consistently use American English spelling

Since the Spring Framework uses American English spelling, this commit
updates Javadoc and the reference manual to ensure consistency in that
regard. However, there are two exceptions to this rule that arise due
to their use within a technical context.

- We use "cancelled/cancelling" instead of "canceled/canceling" in
  numerous places (including error messages).
- We use "implementor" instead of "implementer".

Closes gh-36470

spring-web/src/test/java/org/springframework/web/util/pattern/PathPatternTests.java

@@ -655,7 +655,7 @@ class PathPatternTests {
 		assertMatches(pp,"/");
 		assertMatches(pp,"//");
 
-		// Confirming AntPathMatcher behaviour:
+		// Confirming AntPathMatcher behavior:
 		assertThat(new AntPathMatcher().match("/{foo}", "/")).isFalse();
 		assertThat(new AntPathMatcher().match("/{foo}", "/a")).isTrue();
 		assertThat(new AntPathMatcher().match("/{foo}{bar}", "/a")).isTrue();