Upgrade Ruby to version 3.1.4

Note we updated the `mail` gem in commit 103742847, which is necesary
for Ruby 3.1 because it adds the net-smtp dependency. The net-smtp
library was removed from Ruby in Ruby 3.1, and if we don't include it,
we get an error:

```
cannot load such file -- net/smtp (LoadError)
```

We're also updating the Bundler version in the Gemfile.lock so it's the
one included in Ruby 3.1. Without updating it, we get a warning:

```
Calling `DidYouMean::SPELL_CHECKERS.merge!(error_name => spell_checker)'
has been deprecated. Please call `DidYouMean.correct_error(error_nam e,
spell_checker)' instead.
```

Finally, in order to make Capistrano work, we need to add a couple more
changes:

* Make the net-ssh gem compatible with SSL 3.0; done in commit b2eec088b
* Explicitly allow aliases in the `deploy-secrets.yml` file because
  Psych 4.x (included in Ruby 3.1) doesn't load aliases without this
  option

Dockerfile

@@ -1,4 +1,4 @@
-FROM ruby:3.0.6-buster
+FROM ruby:3.1.4-buster
 
 ENV DEBIAN_FRONTEND noninteractive