Fix staticcheck issues

[timoa]

Hi!

After running staticcheck on the project, it funds a few errors:

Deprecated

internal/tokenutil/tokenutil.go:16:19: jwt.StandardClaims is deprecated: Use RegisteredClaims instead for a forward-compatible way to access registered claims in a struct. (SA1019)
internal/tokenutil/tokenutil.go:31:19: jwt.StandardClaims is deprecated: Use RegisteredClaims instead for a forward-compatible way to access registered claims in a struct. (SA1019)

Unused

mongo/mongo.go:76:6: type nullawareDecoder is unused (U1000)
mongo/mongo.go:81:28: func (*nullawareDecoder).DecodeValue is unused (U1000)

Formatting

internal/tokenutil/tokenutil.go:46:16: error strings should not be capitalized (ST1005)
internal/tokenutil/tokenutil.go:59:16: error strings should not be capitalized (ST1005)
internal/tokenutil/tokenutil.go:71:14: error strings should not be capitalized (ST1005)

This PR will fix them, hopefully :)

[amitshekhariitbhu]

Thanks for this PR, I shall review, test everything and then proceed.

[ZeusMode]

This PR should be approved!