Bump bootstrap from 5.0.1 to 5.1.3

Bumps [bootstrap](https://github.com/twbs/bootstrap-rubygem) from 5.0.1 to 5.1.3.
- [Release notes](https://github.com/twbs/bootstrap-rubygem/releases)
- [Changelog](https://github.com/twbs/bootstrap-rubygem/blob/master/CHANGELOG.md)
- [Commits](https://github.com/twbs/bootstrap-rubygem/compare/v5.0.1...v5.1.3)

---
updated-dependencies:
- dependency-name: bootstrap
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile

@@ -39,7 +39,7 @@ gem 'sdoc', '~> 2.2.0',          group: :doc
 gem 'web-console', '~> 4.2.0', group: :development
 gem 'listen', '~> 3.7.0', group: :development
 
-gem 'bootstrap', '~> 5.0.1'
+gem 'bootstrap', '~> 5.1.3'
 gem "bootstrap_form", "~> 4.5"
 gem 'font-awesome-sass', '~> 5.15.1'