Bump bootstrap from 4.5.3 to 5.1.0

Bumps [bootstrap](https://github.com/twbs/bootstrap-rubygem) from 4.5.3 to 5.1.0.
- [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/v4.5.3...v5.1.0)

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

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

Gemfile.lock

@@ -21,15 +21,15 @@ GEM
       tzinfo (~> 1.1)
     addressable (2.8.0)
       public_suffix (>= 2.0.2, < 5.0)
-    autoprefixer-rails (10.1.0.0)
-      execjs
+    autoprefixer-rails (10.3.3.0)
+      execjs (~> 2)
     babel-source (5.8.35)
     babel-transpiler (0.7.0)
       babel-source (>= 4.0, < 6)
       execjs (~> 2.0)
-    bootstrap (4.5.3)
+    bootstrap (5.1.0)
       autoprefixer-rails (>= 9.1.0)
-      popper_js (>= 1.14.3, < 2)
+      popper_js (>= 2.9.3, < 3)
       sassc-rails (>= 2.0.0)
     builder (3.2.4)
     colorator (1.1.0)
@@ -40,7 +40,7 @@ GEM
       http_parser.rb (~> 0.6.0)
     erubi (1.10.0)
     eventmachine (1.2.7)
-    execjs (2.7.0)
+    execjs (2.8.1)
     extras (0.3.0)
       forwardable-extended (~> 2.5)
     fastimage (2.2.0)
@@ -110,7 +110,7 @@ GEM
       racc (~> 1.4)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
-    popper_js (1.16.0)
+    popper_js (2.9.3)
     public_suffix (4.0.6)
     racc (1.5.2)
     rack (2.2.3)