Update nu-validator from 20.x to 26.x (#1499)

Also fix incorrect heading level in two Pkldoc tests, as this issue is
now flagged by nu-validator.

gradle/libs.versions.toml

@@ -5,6 +5,7 @@ clikt = "5.+"
 commonMark = "0.+"
 downloadTaskPlugin = "5.6.0"
 geantyref = "1.+"
+#noinspection UnusedVersionCatalogEntry
 googleJavaFormat = "1.35.0"
 # must not use `+` because used in download URL
 # 23.1.x requires JDK 20+
@@ -46,11 +47,11 @@ kotlinxSerialization = "1.8.1"
 kotlinxCoroutines = "1.+"
 ktfmt = "0.53"
 # replaces nuValidator's log4j dependency
-# something related to log4j-1.2-api is apparently broken in 2.17.2
+# 2.17.1 is the last version compatible with nuValidator
 log4j = "2.17.1"
 msgpack = "0.9.8"
 nexusPublishPlugin = "2.0.0"
-nuValidator = "20.+"
+nuValidator = "26.+"
 paguro = "3.+"
 shadowPlugin = "9.+"
 slf4j = "1.+"
@@ -82,6 +83,7 @@ jsr305 = { group = "com.google.code.findbugs", name = "jsr305", version.ref = "j
 junitApi = { group = "org.junit.jupiter", name = "junit-jupiter-api", version.ref = "junit" }
 junitEngine = { group = "org.junit.jupiter", name = "junit-jupiter-engine", version.ref = "junit" }
 junitParams = { group = "org.junit.jupiter", name = "junit-jupiter-params", version.ref = "junit" }
+#noinspection UnusedVersionCatalogEntry
 junitLauncher = { group = "org.junit.platform", name = "junit-platform-launcher", version.ref = "junit" }
 kotlinPlugin = { group = "org.jetbrains.kotlin", name = "kotlin-gradle-plugin", version.ref = "kotlin" }
 kotlinPoet = { group = "com.squareup", name = "kotlinpoet", version.ref = "kotlinPoet" }