DEV: Bump required Ruby version to 3.4 (#37819)

Now that our base images ship Ruby 3.4, our Gemfile should require it as
the minimum allowed version.

Gemfile

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-ruby "~> 3.3"
+ruby "~> 3.4"
 
 source "https://rubygems.org"
 # if there is a super emergency and rubygems is playing up, try

Gemfile.lock

@@ -1276,7 +1276,7 @@ CHECKSUMS
   zendesk_api (1.38.0.rc1) sha256=9ebe2575d223ed1c0651b2d10df6c010141cda49d524c38532b472f7e8bb3f7a
 
 RUBY VERSION
-   ruby 3.3.1p55
+   ruby 3.4.7p58
 
 BUNDLED WITH
    2.6.4

README.md

@@ -43,7 +43,7 @@ To get your environment set up, follow one of the setup guides:
 - [Ubuntu/Debian](https://meta.discourse.org/t/14727)
 - [Windows](https://meta.discourse.org/t/75149)
 
-Before you get started, ensure you have the following minimum versions: [Ruby 3.3+](https://www.ruby-lang.org/en/downloads/), [PostgreSQL 13](https://www.postgresql.org/download/), [Redis 7](https://redis.io/download).
+Before you get started, ensure you have the following minimum versions: [Ruby 3.4+](https://www.ruby-lang.org/en/downloads/), [PostgreSQL 13](https://www.postgresql.org/download/), [Redis 7](https://redis.io/download).
 
 For more information, check out [the Developer Documentation](https://meta.discourse.org/c/documentation/developer-guides/56).