Update --java_language_version docs.

PiperOrigin-RevId: 597246684
Change-Id: Ib22725dee8bbb83c7c7e3a1d49da0129779651c3

site/en/docs/user-manual.md

diff --git a/site/en/docs/user-manual.md b/site/en/docs/user-manual.md
index 9bd922b..591e3f8 100644
--- a/site/en/docs/user-manual.md
+++ b/site/en/docs/user-manual.md
@@ -325,7 +325,7 @@
 
 compiles and allows only constructs compatible with Java 8 specification.
 Default value is 11. -->
-Possible values are: 8,  9, 10, 11, 14, and 15  and may be extended by
+Possible values are: 8, 9, 10, 11, 14, 15, and 21 and may be extended by
 registering custom Java toolchains using `default_java_toolchain`.
 
 #### `--tool_java_language_version={{ "<var>" }}version{{ "</var>" }}` {:#tool-java-language-version}