As of Bundler v2.3 (when using RubyGems v3.3+), Bundler will automatically use the BUNDLED WITH version specified in Gemfile.lock when running the bundle install command. If the specified version of Bundler is not available, it will be downloaded and installed.

$ bundler -v
Bundler version 2.3.15

$ tail -2 ./example/Gemfile.lock
BUNDLED WITH
   2.1.4

$ (cd ./example && bundler -v)
Bundler version 2.1.4

If you can't upgrade to v2.3, there is a workaround. You will have to install the correct version of Bundler manually, but then you can use the _version_ syntax.

$ bundler _2.1.4_ -v
Bundler version 2.1.4