Switch to "ListProjectMergeRequests" for GitLab

[jrryjcksn]

Description

The support for listing GitLab pull requests (merge requests) is currently using ListMergeRequests. This returns all merge requests accessible by the authenticated user. We should instead be using ListProjectMergeRequests which scopes the query to a particular project (repository).

Test results

arete: ~jj/go-git-providers> CGO_ENABLED=1 make test
go mod tidy
go fmt ./...
go vet ./...
go test  -race -coverprofile=coverage.txt -covermode=atomic ./...
?   	github.com/fluxcd/go-git-providers/bitbucket	[no test files]
ok  	github.com/fluxcd/go-git-providers/github	18.484s	coverage: 49.1% of statements
ok  	github.com/fluxcd/go-git-providers/gitlab	53.498s	coverage: 73.1% of statements
ok  	github.com/fluxcd/go-git-providers/gitprovider	0.038s	coverage: 92.2% of statements
?   	github.com/fluxcd/go-git-providers/gitprovider/cache	[no test files]
?   	github.com/fluxcd/go-git-providers/gitprovider/testutils	[no test files]
ok  	github.com/fluxcd/go-git-providers/stash	7.006s	coverage: 70.0% of statements
ok  	github.com/fluxcd/go-git-providers/validation	0.033s	coverage: 100.0% of statements
arete: ~jj/go-git-providers> 

[yiannistri]

LGTM 👍