fix: bundler requires Ruby version >=3.0.0 (#70)

The last version of bundler (>= 0) to support your Ruby & RubyGems
was 2.4.22. Try installing it with gem install bundler -v 2.4.22
bundler requires Ruby version >= 3.0.0. The current ruby version is
2.7.8.225.

README.md

@@ -104,7 +104,7 @@ jobs:
           jekyll_src: './'           # Default is root directory
           jekyll_cfg: '_config.yml'  # Default is _config.yml
           jekyll_baseurl: ''         # Default is according to _config.yml
-          bundler_ver: '>=0'         # Default is latest bundler version
+          bundler_ver: '~>2.4.0'     # Default is latest bundler version
           cname: ''                  # Default is to not use a cname
           actor: ''                  # Default is the GITHUB_ACTOR
           pre_build_commands: ''     # Installing additional dependencies (Arch Linux)

entrypoint.sh

@@ -12,7 +12,7 @@ SSH_PRIVATE_KEY=${INPUT_SSH_PRIVATE_KEY:=}
 ACTOR=${INPUT_ACTOR}
 REPOSITORY=${INPUT_REPOSITORY}
 BRANCH=${INPUT_BRANCH}
-BUNDLER_VER=${INPUT_BUNDLER_VER:=>=0}
+BUNDLER_VER=${INPUT_BUNDLER_VER:=~>2.4.0}
 JEKYLL_SRC=${INPUT_JEKYLL_SRC:=./}
 JEKYLL_CFG=${INPUT_JEKYLL_CFG:=./_config.yml}
 JEKYLL_BASEURL=${INPUT_JEKYLL_BASEURL:=}