chore(build): update download center config with new release info, instead of replacing it entirely when doing release - MONGOSH-1348

[himanshusinghs]

This PR updates our method of publishing download center config. Earlier we used to replace the download center config entirely with the to-be released version. With this change, we instead will be taking into account the already published config, if any, and will insert the version info for the to-be released version while keeping the other, earlier released major versions.

Pointing this to main only because it does not block us from releasing main as of now for 1.x.x

Earlier

Existing Config	Current Release Version	Updated Config
{ versions: [{ _id_: '1.2.2' version: '1.2.2', platform: [{ ... }] }], ... }	1.2.3	{ versions: [{ _id_: '1.2.3' version: '1.2.3', platform: [{ ... }] }], ... }
{ versions: [{ _id_: '1.2.2' version: '1.2.2', platform: [{ ... }] }], ... }	2.0.0	{ versions: [{ _id_: '2.0.0' version: '2.0.0', platform: [{ ... }] }], ... }

Now

Existing Config	Current Release Version	Updated Config
{ versions: [{ _id_: '1.2.2' version: '1.2.2', platform: [{ ... }] }], ... }	1.2.3	{ versions: [{ _id_: '1.2.3' version: '1.2.3', platform: [{ ... }] }], ... }
{ versions: [{ _id_: '1.2.2' version: '1.2.2', platform: [{ ... }] }], ... }	2.0.0	{ versions: [{ _id_: '1.2.2' version: '1.2.2', platform: [{ ... }] }, { _id_: '2.0.0' version: '2.0.0', platform: [{ ... }] }], ... }
{ versions: [{ _id_: '1.2.2' version: '1.2.2', platform: [{ ... }] }, { _id_: '2.0.0' version: '2.0.0', platform: [{ ... }] }], ... }	2.1.0	{ versions: [{ _id_: '1.2.2' version: '1.2.2', platform: [{ ... }] }, { _id_: '2.1.0' version: '2.1.0', platform: [{ ... }] }], ... }

[addaleax]

Nice work!