upgrade docs job to 8.4

Author: bshaffer

.github/workflows/docs.yaml

@@ -21,7 +21,7 @@ jobs:
     - name: Setup PHP
       uses: shivammathur/setup-php@verbose
       with:
-        php-version: "8.1"
+        php-version: "8.4"
     - name: Extract phpDocumentor
       id: extract
       uses: shrink/actions-docker-extract@v2

.kokoro/docs/docker/Dockerfile

@@ -1,7 +1,7 @@
 FROM gcr.io/gcp-runtimes/ubuntu_20_0_4
 ENV PHP_DIR=/opt/php80
-ENV PHP_VERSION=8.1.18
-ENV PHP_SRC_DIR=/usr/local/src/php81-build
+ENV PHP_VERSION=8.4.14
+ENV PHP_SRC_DIR=/usr/local/src/php84-build
 ENV PATH=${PATH}:/usr/local/bin:${PHP_DIR}/bin
 
 RUN apt-get update && \
@@ -36,7 +36,7 @@ RUN apt-get update && \
 # Remove old version of PHP
 RUN apt purge -y php7.0-common
 
-# Install PHP 8.1
+# Install PHP 8.4
 RUN ln -s /usr/lib/libc-client.a /usr/lib/x86_64-linux-gnu/libc-client.a && \
     mkdir -p ${PHP_DIR} ${PHP_SRC_DIR} ${PHP_DIR}/lib/conf.d && \
     cd ${PHP_SRC_DIR} && \

dev/src/DocFx/Node/XrefTrait.php

@@ -30,7 +30,7 @@ trait XrefTrait
      *
      * @see https://github.com/protocolbuffers/protobuf/blob/00a9043d0d79b65b47a048a20d01b16618c8396a/src/google/protobuf/compiler/php/names.cc#L18-L34
      */
-    const RESERVED_NAMES = [
+    private const RESERVED_NAMES = [
         'abstract',     'and',        'array',        'as',         'break',
         'callable',     'case',       'catch',        'class',      'clone',
         'const',        'continue',   'declare',      'default',    'die',