npm downgraded to 5.x if default version is 10.x

[sdaicz]

Describe the bug
Now that the default npm version for node 20.7 is 10.1.0, if an npm version is not specified it will be incorrectly downgraded to 5.x

To Reproduce
Steps to reproduce the behavior:

1. Specify "engines": { "node": "20.x" } in your package.json file (note that "npm" is not specified)

2. Build your app on Heroku

3. In the build log you'll see something like:

   Downloading and installing node 20.7.0...
   Detected package-lock.json: defaulting npm to version 5.x.x
   Bootstrapping npm 5.x.x (replacing 10.1.0)...
   npm 5.10.0 installed
   

4. Your build will probably fail with some obscure error message later

Versions (please complete the following information):

• Heroku Stack: heroku-22
• Node Version: 20.7.0
• NPM Version: 10.1.0
• Buildpack Version: heroku/nodejs v221

Additional context
The issue originates from this line, which only looks at the first character of the default node version:

heroku-buildpack-nodejs/lib/binaries.sh

Line 115 in c14b37d

if $npm_lock && [ "$version" == "" ] && [ "${npm_version:0:1}" -lt "5" ]; then

[davekaro]

Similarly, I noticed some new warnings present that are due to the same bug in the code.

 !     This version of npm (10.1.0) does not support package-lock.json. Please
       update your npm version in package.json.
       https://devcenter.heroku.com/articles/nodejs-support#specifying-an-npm-version

which comes from

heroku-buildpack-nodejs/lib/failure.sh

Line 760 in c14b37d

if $npm_lock && [ "${npm_version:0:1}" -lt "5" ]; then

.

I searched the code and found it may also present a problem here

heroku-buildpack-nodejs/lib/failure.sh

Line 748 in c14b37d

if [ "${npm_version:0:1}" -lt "2" ]; then