Add GPG signing + remove some triggers in Update References workflow

[NikitaSkrynnik]

Descrption

To make GPG signing work we need to:

1. Create gpg keys with a passphrase, name == NSMBot, email == nsmbot@networkservicmesh.io

gpg --full-gen-key

2. Export public and private keys

gpg --export -a nsmbot@networkservicmesh.io > public.gpg
gpg --export-secret-key -a nsmbot@networkservicmesh.io > private.gpg

3. Add the public key to NSMBot GPG keys on Github
4. Add the private key and passphrase to deployments-k8s secrets GPG_PRIVATE_KEY and GPG_PASSPHRASE

Issue link

#11593

How Has This Been Tested?

• Added unit testing to cover
• Tested manually
• Tested by integration testing
• Have not tested

Types of changes

• Bug fix
• New functionality
• Documentation
• Refactoring
• CI

[denis-tingaikin]

@edwarnicke Please have a look at the suggested solution when you get a chance ☝️
Let us know when the required secrets for the NSMBot will be available or if you have any questions.