Sign with the main key, not subkey (#3023)

## Description
The keyring uploaded to buddy had some subkeys and the keygrip used in dist-build.sh was for a subkey. While there are no errors while signing, centos/rhel < 8 has an older rpm (4.11.3) which cannot process subkeys.

## Related Issue
TykTechnologies/internal#77

## Motivation and Context
Customers on centos 7 and who care about the signature have complained.

## How This Has Been Tested
1. Built a clean centos 7 image and installed a package signed with the old key
1. Updated `tyk-tyk_gateway.repo` to use the new pubkey
1. Update the package to a new version signed by the new key

## Screenshots (if appropriate)

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to change)

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes that apply -->
<!-- If you're unsure about any of these, don't hesitate to ask; we're here to help! -->
- [x] Make sure you are requesting to **pull a topic/feature/bugfix branch** (right side). If pulling from your own
      fork, don't request your `master`!
- [x] Make sure you are making a pull request against the **`master` branch** (left side). Also, you should start
      *your branch* off *our latest `master`*.
- [ ] My change requires a change to the documentation.
  - [ ] If you've changed APIs, describe what needs to be updated in the documentation.
- [ ] I have updated the documentation accordingly.
- [ ] Modules and vendor dependencies have been updated; run `go mod tidy && go mod vendor`
- [ ] I have added tests to cover my changes.
- [x] All new and existing tests passed.
- [ ] Check your code additions will not fail linting checks:
  - [ ] `go fmt -s`
  - [ ] `go vet`

## Deployment checklist
- [ ] gw unstable
- [ ] gw stable
- [ ] db unstable
- [ ] db stable
- [ ] pump stable
- [ ] pump unstable

README.signatures

@@ -4,15 +4,20 @@ tl;dr
 RHEL does not support signing RPMs with subkeys. This is supposedly
 fixed in RHEL 8. Thus, we use the same key to sign rpms and debs.
 
+It is best to have a key with _no_ subkeys as RPM seems to process
+these type of keyrings incorrectly.
+
 These keys DO NOT expire.
 
-Fingerprints:
+Keygrip:
 Main key                : 12B5D62C28F57592D1575BD51ED14C59E37DAC20
-Signing subkey (deb)    : 9ADE11DA6DD70355E7C1C270543ABE02AC6AC40A
-Signing subkey (unused) : 9A4FA354841E41E39A49A461B34B3EF69DCBD7FE
 
-These fingerprints above should match between this document and your
-keyring. The other numbers in the gpg output will probably not match.
+This fingerprints above should match between this document and your
+keyring. The other numbers in the gpg output below will probably not
+match.
+
+This keygrip corresponds to the cache_id for the gpg passphrase
+seeding.
 
 Verification
 ============

bin/dist_build.sh

@@ -4,7 +4,7 @@ set -ex
 
 : ${ORGDIR:="/go/src/github.com/TykTechnologies"}
 : ${SOURCEBINPATH:="${ORGDIR}/tyk"}
-: ${SIGNKEY:="9ADE11DA6DD70355E7C1C270543ABE02AC6AC40A"}
+: ${SIGNKEY:="12B5D62C28F57592D1575BD51ED14C59E37DAC20"}
 : ${BUILDPKGS:="1"}
 : ${SIGNPKGS:="1"}
 : ${PKGNAME:="tyk-gateway"}