Treat GMT as a legacy alias of UTC (#16292)

Support GMT as a valid time zone identifier, even without a zoneinfo database. Historically, it has been used as a generic name in a similar manner as UTC.
As the use case in #16277 shows, GMT is relevant for data exchange.

Author: straight-shoota

spec/std/time/location_spec.cr

@@ -103,6 +103,24 @@ class Time::Location
           Location.load("").should eq Location::UTC
           Location.load("Etc/UTC").should eq Location::UTC
         end
+
+        with_zoneinfo("nonexistent_zipfile.zip") do
+          Location.load("UTC").should eq Location::UTC
+          Location.load("").should eq Location::UTC
+          Location.load("Etc/UTC").should eq Location::UTC
+        end
+      end
+
+      it "treats GMT as special case" do
+        with_zoneinfo do
+          Location.load("GMT").should eq Location::UTC
+          Location.load("Etc/GMT").should eq Location::UTC
+        end
+
+        with_zoneinfo("nonexistent_zipfile.zip") do
+          Location.load("GMT").should eq Location::UTC
+          Location.load("Etc/GMT").should eq Location::UTC
+        end
       end
 
       describe "validating name" do

src/time/location.cr

@@ -320,7 +320,7 @@ class Time::Location
   # :nodoc:
   def self.load?(name : String, & : String? ->) : Location?
     case name
-    when "", "UTC", "Etc/UTC"
+    when "", "UTC", "Etc/UTC", "GMT", "Etc/GMT"
       # `UTC` is a special identifier, empty string represents a fallback mechanism.
       # `Etc/UTC` is technically a tzdb identifier which could potentially point to anything.
       # But we map it to `Location::UTC` directly for convenience which allows it to work