tools: use latest rather than next for markdown linting plugin

eslint-plugin-markdown is being installed @next to get a bugfix for
eslint/markdown#69 but that
bugfix is in 1.0.0. Go back to installing @latest rather than @next.

PR-URL: #26345
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

tools/update-eslint.sh

@@ -16,7 +16,7 @@ npm init --yes
 npm install --global-style --no-bin-links --production --no-package-lock eslint@latest
 cd node_modules/eslint
 
-npm install --no-bin-links --production --no-package-lock eslint-plugin-markdown@next
+npm install --no-bin-links --production --no-package-lock eslint-plugin-markdown@latest
 cd ../..
 
 # Use dmn to remove some unneeded files.