Fix Grant revocation

[jabbrwcky]

If the grant is already gone when the reconciler tries to revoke it,
it should mark the resource as successfully removed, not error.

MySQL 8.x+ supports
REVOKE %s IF EXISTS ON %s.%s FROM %s@%s IGNORE UNKNOWN USER
to avoid an error (which would be a cleaner solution IMO),
but this will only be possible when dropping support for earlier versions.

Description of your changes

Checks the actual mysql error code that is returned when the grant
revocation SQL errors; if the code states that the grant does not exist,
it returns nil for successful removal of the grant.

Basically uses the same code as

provider-sql/pkg/controller/mysql/grant/reconciler.go

Lines 227 to 231 in ed6c55c

	var myErr *mysqldriver.MySQLError
	if errors.As(err, &myErr) && myErr.Number == errCodeNoSuchGrant {
	// The user doesn't (yet) exist and therefore no grants either
	return nil, &managed.ExternalObservation{ResourceExists: false}, nil
	}

Fixes #47

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable to ensure this PR is ready for review.

How has this code been tested

Added unit test for this case to grants/reconciler_test.go

[chlunde]

Nice, could you git commit -s? See https://github.com/crossplane-contrib/provider-sql/pull/118/checks?check_run_id=9415901160

Just a heads up: I also think we will need a rebase when #105 is merged

[jabbrwcky]

Commit is signed

[Duologic]

Can you fix the lint error?

[jabbrwcky]

Fixed the reported linter issues.

You might consider upgrading golangci-lint (if possible for github-actions?), the output from v 1.50.1 is much more helpful:

pkg/controller/mysql/grant/reconciler.go:333:4: commentFormatting: put a space between `//` and comment text (gocritic)
                        //MySQL automatically deletes related grants if the user has been deleted