gem: pin sassc to 2.1

ideally we'd remove sassc as we make no use of it but in the meantime:
sass/sassc-ruby#189 (comment)

Author: freesteph

Gemfile

@@ -22,6 +22,7 @@ gem "mini_magick"
 gem "pdfkit"
 gem "pg", ">= 0.18", "< 2.0"
 gem "puma", "~> 4"
+gem "sassc", "~> 2.1.0"
 gem "rails", "~> 6.1.4"
 gem "rswag-ui"
 gem "webpacker", "~> 5.4"

Gemfile.lock

@@ -390,6 +390,10 @@ GEM
       ffi (~> 1.12)
     ruby2_keywords (0.0.5)
     rubyzip (2.3.2)
+    sassc (2.1.0)
+      ffi (~> 1.9)
+    sassc (2.1.0-x86_64-linux)
+      ffi (~> 1.9)
     selenium-webdriver (3.142.7)
       childprocess (>= 0.5, < 4.0)
       rubyzip (>= 1.2.2)
@@ -490,6 +494,7 @@ DEPENDENCIES
   rubocop
   rubocop-rails
   rubocop-rspec
+  sassc (~> 2.1.0)
   selenium-webdriver
   simplecov
   web-console