Marks the RemoteTeamId field of RemoteClusters as deprecated (#27364)

* Marks the RemoteTeamId field of RemoteClusters as deprecated

The `RemoteTeamId` was used both in the `RemoteCluster` model and as
part of remote invites. It existed so two different remotes could have
multiple secure connections between them, and have each of those
connections scoped to a team, sharing through each only the channels
that belong to their corresponding team.

The way that we're thinking on the feature currently only contemplates
one secure connection between two servers, and shares all the
channels through that secure connection, so this field is no longer
needed.

As we don't have a system in place for the user to choose in which
team a channel should be created from an invite, this change adds a
mechanism that checks the invite for a teamId, and if it's not
present, fetches a team from the database to create the channel
into. This makes the change backwards compatible for secure
connections that already have an established behavior and allows us to
move forward with the implementation of an alternative.

* Mark invite teamId field as deprecated

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/platform/services/sharedchannel/channelinvite.go

@@ -18,7 +18,7 @@ import (
 // channelInviteMsg represents an invitation for a remote cluster to start sharing a channel.
 type channelInviteMsg struct {
 	ChannelId            string            `json:"channel_id"`
-	TeamId               string            `json:"team_id"`
+	TeamId               string            `json:"team_id"` // Deprecated: this field is no longer used. It's only kept for backwards compatibility.
 	ReadOnly             bool              `json:"read_only"`
 	Name                 string            `json:"name"`
 	DisplayName          string            `json:"display_name"`
@@ -52,7 +52,6 @@ func (scs *Service) SendChannelInvite(channel *model.Channel, userId string, rc
 
 	invite := channelInviteMsg{
 		ChannelId:   channel.Id,
-		TeamId:      rc.RemoteTeamId,
 		ReadOnly:    sc.ReadOnly,
 		Name:        channel.Name,
 		DisplayName: sc.ShareDisplayName,
@@ -256,9 +255,21 @@ func (scs *Service) handleChannelCreation(invite channelInviteMsg, rc *model.Rem
 		return scs.createDirectChannel(invite, rc)
 	}
 
+	teamId := invite.TeamId
+	// if the invite doesn't have a teamId associated and until the
+	// acceptance of an invite includes selecting a team, we use the
+	// first team of the list
+	if teamId == "" {
+		teams, err := scs.server.GetStore().Team().GetAllPage(0, 1, nil)
+		if err != nil {
+			return nil, fmt.Errorf("cannot get team to create the channel `%s`: %w", invite.ChannelId, err)
+		}
+		teamId = teams[0].Id
+	}
+
 	channelNew := &model.Channel{
 		Id:          invite.ChannelId,
-		TeamId:      invite.TeamId,
+		TeamId:      teamId,
 		Type:        invite.Type,
 		DisplayName: invite.DisplayName,
 		Name:        invite.Name,