Apply lower bound ruby-lsp version constraint in composed bundle

When the composed `.ruby-lsp/Gemfile` is generated for users who don't
have `ruby-lsp` in their own Gemfile, apply a `>= 0.18.0` lower bound.

Without this constraint, Bundler could resolve to very old versions
that cause errors. Reasons for choosing 0.18.0:

- Fixes resolution to versions like 0.11.2 which cause `NameError:
  uninitialized constant RubyLsp::RuboCop::Cop`
- First version with stable Prism 1.0 dependency (`~> 1.0`) — earlier
  versions used narrow pre-1.0 pins that caused dependency resolution
  issues
- Includes `rbs >= 3` dependency (added in v0.17.3) needed for core
  class indexing
- Not overly restrictive — v0.18.0 is from September 2024, leaving
  ~18 months of versions available

Author: st0012

lib/ruby_lsp/setup_bundler.rb

@@ -34,6 +34,7 @@ def stdout
     # Gems that should be kept up to date in the composed bundle. When updating, any of these gems that are not
     # already in the user's Gemfile will be updated together.
     GEMS_TO_UPDATE = ["ruby-lsp", "debug", "prism", "rbs"].freeze #: Array[String]
+    RUBY_LSP_MIN_VERSION = "0.18.0" #: String
 
     #: (String project_path, **untyped options) -> void
     def initialize(project_path, **options)
@@ -170,9 +171,8 @@ def write_custom_gemfile
       end
 
       unless @dependencies["ruby-lsp"]
-        ruby_lsp_entry = +'gem "ruby-lsp"'
-        ruby_lsp_entry << ", \">= 0.a\"" if @beta
-        ruby_lsp_entry << ", require: false, group: :development"
+        version = @beta ? "0.a" : RUBY_LSP_MIN_VERSION
+        ruby_lsp_entry = +"gem \"ruby-lsp\", \">= #{version}\", require: false, group: :development"
         ruby_lsp_entry << ", github: \"Shopify/ruby-lsp\", branch: \"#{@branch}\"" if @branch
         parts << ruby_lsp_entry
       end