use int64 instead of int for metadata version

[arbll]

Release Notes: Metadata version now uses int64

Types of changes:

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Description of the changes being introduced by the pull request:

We have a high write throughput on our TUF repositories and there's a good chance we'll reach 2B+ snapshots versions in the future which would break our clients on 32bit archs. Let's standardize to int64 to avoid any surprises.

[coveralls]

Pull Request Test Coverage Report for Build 2045946320

• 14 of 21 (66.67%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage remained the same at 71.277%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
verify/verify.go	3	4	75.0%
repo.go	6	9	66.67%
util/util.go	1	4	25.0%

Files with Coverage Reduction	New Missed Lines	%
util/util.go	1	40.84%

Totals
Change from base Build 2029695489:	0.0%
Covered Lines:	2278
Relevant Lines:	3196

---

💛 - Coveralls

[arbll]

@asraa @ethan-lowman-dd Gentle bump, let me know if there's anything blocking this. Our backend can wait but we'd like to include it asap in our clients

[ethan-lowman-dd]

LGTM. I believe the goreleaser configuration now requires conventional commit messages with annotations for breaking changes. Could you rebase and edit the commit message with format described here? https://www.conventionalcommits.org/en/v1.0.0/