adding SAML2024 example certs + references, updating tests to use them

[bleachbyte]

🎫 Ticket

https://gitlab.login.gov/lg/identity-devops/-/issues/4387

NOTE: As of the opening of this PR, saml2024.crt and saml2024.key.enc files have been created for all environments, with the appropriate suffix / secret_key_passphrase values added to saml_endpoint_configs in each application.yml file for said environments (including int / staging / dm / prod).

🛠 Summary of changes

• Adds 2024 suffix in saml_endpoint_configs default config + example saml2024 cert files
• Adds cert files to config/initializers/app_artifacts.rb
• Updates spec tests to check the 2024 endpoint/artifacts/etc., with the endpoint_configs test also checking 2023 instead of 2022

NOTE: Once all partners support/are utilizing the 2024 SAML certs, a follow-up PR will be created to remove all references to the 2023 certs.

📜 Testing Plan

1. Created saml2024.crt and saml2024.key.enc files for my bleachbyte environment (built from dev configs) + added to S3 / application.yml
2. Pushed up current code to my environment + built new idp hosts, verifying that hosts built properly
3. Verified that saml2023 and saml2024 endpoints are available

Relevant logs display entries for saml2023 and saml2024 cert / key files. Site runs properly and endpoints are valid and reachable.