Update theme gem and document PLATFORM_NAME variable

Why these changes are being introduced:

The GDT project added a feature to the theme gem to conditionally
include the platform name in the header if the `PLATFORM_NAME`
environment variable is supplied.

Relevant ticket(s):

https://mitlibraries.atlassian.net/browse/GDT-124

How this addresses that need:

This updates the theme gem to the v1.2 tag, which includes the new
feature, and documents the PLATFORM_NAME variable.

Side effects of this change:

None. The app will continue to use the standard header if
PLATFORM_NAME is not supplied.

Gemfile

@@ -9,7 +9,7 @@ gem 'graphql-client'
 gem 'http'
 gem 'importmap-rails'
 gem 'jbuilder'
-gem 'mitlibraries-theme', git: 'https://github.com/mitlibraries/mitlibraries-theme', tag: 'v1.1'
+gem 'mitlibraries-theme', git: 'https://github.com/mitlibraries/mitlibraries-theme', tag: 'v1.2'
 gem 'puma'
 gem 'rails', '~> 7.0'
 gem 'sentry-rails'

Gemfile.lock

@@ -1,7 +1,7 @@
 GIT
   remote: https://github.com/mitlibraries/mitlibraries-theme
-  revision: 93b931b802485f9e35a6878f957b3fd88ae3b294
-  tag: v1.1
+  revision: bcbe5d3de36a92d275085a045c5c4d8f30f33e62
+  tag: v1.2
   specs:
     mitlibraries-theme (1.0.2)
       rails (>= 6, < 8)

README.md

@@ -56,6 +56,7 @@ mode (e.g., `GDT=false` will still enable GDT features). Note that this is curre
 may have unexpected consequences if applied to other TIMDEX UI apps.
 - `GLOBAL_ALERT`: The main functionality for this comes from our theme gem, but when set the value will be rendered as
   safe html above the main header of the site.
+- `PLATFORM_NAME`: The value set is added to the header after the MIT Libraries logo. The logic and CSS for this comes from our theme gem.
 - `SENTRY_DSN`: Client key for Sentry exception logging.
 - `SENTRY_ENV`: Sentry environment for the application. Defaults to 'unknown' if unset.
 - `TIMDEX_INDEX`: Name of the index, or alias, to provide to the GraphQL endpoint. Defaults to `nil` which will let TIMDEX determine the best index to use. Wildcard values can be set, for example `rdi*` would search any indexes that begin with `rdi` in the underlying OpenSearch instance behind TIMDEX.