Use bundler v2 in development (#1463)

Prior to this commit, the `Gemfile.lock` enforced the use of
Bundler v1.17.3 which was released in 2014.

On Ruby 3, this triggers the following deprecation warning:

```
/home/user/.rvm/gems/ruby-3.0.2@shoulda-matchers/gems/bundler-1.17.3/lib/bundler/shared_helpers.rb:118: warning: Pathname#untaint is deprecated and will be removed in Ruby 3.2.
```

Since the gem itself in v5.0.0 requires at least Ruby v2.6,
it seems fair to require a recent version of Bundler v2,
which in turn runs of every Ruby v2.3 or later.

bin/setup

@@ -8,8 +8,6 @@ set -euo pipefail
 # section. You may also delete this section altogether if your project doesn't
 # need a custom setup.
 
-USE_BUNDLER_1=1
-
 provision-project() {
   banner "Installing Appraisals"
   bundle exec appraisal install
@@ -187,8 +185,6 @@ setup() {
 # --- RUBY ---------------------------------------------------------------------
 
 provision-ruby() {
-  USE_BUNDLER_1=0
-
   if [[ -f .tool-versions ]]; then
     REQUIRED_RUBY_VERSION=$(cat .tool-versions | grep '^ruby ' | head -n 1 | sed -Ee 's/^ruby (.+)$/\1/')
   elif [[ -f .ruby-version ]]; then
@@ -265,11 +261,7 @@ has-bundler() {
 ensure-project-ruby-dependencies-installed() {
   banner 'Installing Ruby dependencies'
 
-  if [[ $USE_BUNDLER_1 -eq 1 ]] && (! has-bundler || ! [[ $(bundle -v) =~ '^Bundler version 1\.' ]]); then
-    gem install bundler:'~> 1.0'
-  elif ! has-bundler; then
-    gem install bundler
-  fi
+  gem install bundler
 
   bundle check || bundle install
 }