fix: format updated_at as UNIX time stamp

[hperl]

In compliance with https://openid.net/specs/openid-connect-core-1_0.html#StandardClaims

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

Merging #3631 (40183fb) into master (d1f9ba8) will decrease coverage by 0.09%.
Report is 2 commits behind head on master.
The diff coverage is 53.84%.

❗ Current head 40183fb differs from pull request most recent head 1f45354. Consider uploading reports for the commit 1f45354 to get more accurate results

@@            Coverage Diff             @@
##           master    #3631      +/-   ##
==========================================
- Coverage   76.18%   76.09%   -0.09%     
==========================================
  Files         133      133              
  Lines       10043    10003      -40     
==========================================
- Hits         7651     7612      -39     
- Misses       1868     1869       +1     
+ Partials      524      522       -2     

Files Changed	Coverage Δ
oauth2/handler.go	67.62% <53.84%> (-0.33%)	⬇️

... and 2 files with indirect coverage changes

[aeneasr]

Suggested change

	var s string
	switch v := val.(type) {
	case string:
	s = v
	case fmt.Stringer:
	s = v.String()
	default:
	continue
	}
	t, err := time.Parse(time.RFC3339Nano, s)
	t, err := time.Parse(time.RFC3339Nano, fmt.Sprintf("%s", s))

[aeneasr]

Should be fixed in the consent app IMO