build: only write new minAppVersion requirements to versions.json

[johannrichard]

Currently, the version-bump.mjs script will write a new version: minAppVersion pair on every run with a version which is not yet in versions.json. However, we only need to update versions.json if the minAppVersion for the plugin changes. This PR modifies the script such that it only adds a new version requirement if minAppVersion in the current manifest.json is not already in versions.json. This should declutter versions.json.

Important:
This simply checks whether the minAppVersion is part of the versions.json or not. But it does not do any semantic comparisons of the existing versions against the new minAppVersion. This might lead to the (I guess unlikely, but entirely possible) situation that a lower minAppVersion for a newer plugin version is added to versions.json. I can't foresee if Obsidian would handle this gracefully, if that would be expected behaviour anyway, or if it would break things. However, since the current version of version-bump.mjs simply adds whatever minAppVersion the updated manifest.json contains, we won't be changing that fundamental behaviour.