Incorrect milestone

[yp05327]

I think this is caused by that v1.19 does not support any more, so bot can not find the correct milestone.
Maybe we can add a check of this in L14:

gitea-backporter/src/milestones.ts

Lines 10 to 14 in 4dc4c52

	// find the gitea version that matches the PR's base branch
	let giteaVersion = giteaVersions.find((version) =>
	pr.base.ref === `release/v${version.majorMinorVersion}`
	);

[yardenshoham]

Is it correct to open a PR to release/v1.19 if there will not be a v1.19 release?