LG-11422: Provide Sender ID for UK SMS

[aduth]

🎫 Ticket

LG-11422

🛠 Summary of changes

Adds UK to the set of sender ID countries so that the preconfigured sender ID is provided for SMS to the country.

Source for how Sender ID is configured using our single static Sender ID:

identity-idp/config/initializers/telephony.rb

Lines 16 to 19 in a1df492

	c.country_sender_ids = IdentityConfig.store.pinpoint_sms_sender_id.presence &&
	PinpointSupportedCountries::SENDER_ID_COUNTRIES.index_with do
	IdentityConfig.store.pinpoint_sms_sender_id
	end

Reference that Sender ID is supported without registration (see "Yes" under "Supports Sender ID", notably not "Registration required"):

https://docs.aws.amazon.com/pinpoint/latest/userguide/channels-sms-countries.html

Reference that "GB" is the country identifier:

identity-idp/config/pinpoint_supported_countries.yml

Lines 317 to 321 in a1df492

	GB:
	country_code: '44'
	name: United Kingdom
	supports_sms: true
	supports_voice: true

Slack discussion: https://gsa-tts.slack.com/archives/C01710KMYUB/p1698267798949799

📜 Testing Plan

I'm not aware that there's a way to test this.