Summary: Replace the thin grey timeline line created via a 1px image file with modern CSS. According to https://caniuse.com/?search=linear-gradient this should be long supported, except for Safari versions before 2022 which may have some problems (or not). Closes T16202 Refs T15056 Test Plan: Go to a page which exposes the vertical timeline line, e.g. a Maniphest task, and look at it. (Note that depending on browser zoom levels there might be small differences.) Reviewers: O1 Blessed Committers, mainframe98 Reviewed By: O1 Blessed Committers, mainframe98 Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno Maniphest Tasks: T16202, T15056 Differential Revision: https://we.phorge.it/D26229