chore(alpine): add EOL date for Alpine 3.21 (#8221)

Author: chrisnovakovic

docs/docs/coverage/os/index.md

@@ -11,7 +11,7 @@ Trivy supports operating systems for
 
 | OS                                    | Supported Versions                  | Package Managers |
 |---------------------------------------|-------------------------------------|------------------|
-| [Alpine Linux](alpine.md)             | 2.2 - 2.7, 3.0 - 3.20, edge         | apk              |
+| [Alpine Linux](alpine.md)             | 2.2 - 2.7, 3.0 - 3.21, edge         | apk              |
 | [Wolfi Linux](wolfi.md)               | (n/a)                               | apk              |
 | [Chainguard](chainguard.md)           | (n/a)                               | apk              |
 | [Red Hat Enterprise Linux](rhel.md)   | 6, 7, 8                             | dnf/yum/rpm      |

pkg/detector/ospkg/alpine/alpine.go

@@ -48,6 +48,7 @@ var (
 		"3.18": time.Date(2025, 5, 9, 23, 59, 59, 0, time.UTC),
 		"3.19": time.Date(2025, 11, 1, 23, 59, 59, 0, time.UTC),
 		"3.20": time.Date(2026, 04, 1, 23, 59, 59, 0, time.UTC),
+		"3.21": time.Date(2026, 12, 5, 23, 59, 59, 0, time.UTC),
 		"edge": time.Date(9999, 1, 1, 0, 0, 0, 0, time.UTC),
 	}
 )