apacheGH-39737: [Release][Docs] Update post release documentation task (

apache#39762)

This PR updates the `dev/release/post-08-docs.sh` task so that

- `DOCUMENTATION_OPTIONS.theme_switcher_version_match` changes from `""` to `"{previous_version}"` 
- `DOCUMENTATION_OPTIONS.show_version_warning_banner` changes from `false` to `true`

for the documentation that is moved to a subfolder when a new major release is done.
* Closes: apache#39737

Lead-authored-by: AlenkaF <frim.alenka@gmail.com>
Co-authored-by: Alenka Frim <AlenkaF@users.noreply.github.com>
Co-authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

dev/release/post-08-docs.sh

@@ -86,6 +86,21 @@ if [ "$is_major_release" = "yes" ] ; then
 fi
 git add docs
 git commit -m "[Website] Update documentations for ${version}"
+
+# Update DOCUMENTATION_OPTIONS.theme_switcher_version_match and
+# DOCUMENTATION_OPTIONS.show_version_warning_banner
+pushd docs/${previous_series}
+find ./ \
+  -type f \
+  -exec \
+    sed -i.bak \
+      -e "s/DOCUMENTATION_OPTIONS.theme_switcher_version_match = '';/DOCUMENTATION_OPTIONS.theme_switcher_version_match = '${previous_version}';/g" \
+      -e "s/DOCUMENTATION_OPTIONS.show_version_warning_banner = false/DOCUMENTATION_OPTIONS.show_version_warning_banner = true/g" \
+      {} \;
+find ./ -name '*.bak' -delete
+popd
+git add docs/${previous_series}
+git commit -m "[Website] Update warning banner for ${previous_series}"
 git clean -d -f -x
 popd