sqlite3: consistently comment where we want to upgrade.

Now you can grep for 'sqlite3 version' and see where we would like
to update.

Debian 11 (Bullseye) and Ubuntu 20.04 (Focal) ship with SQLite 3.31.1.
RHEL 9 ships with 3.34.1.  Fedora 38+ uses SQLite 3.40+.

Unfortunately, RHEL8 ships with 3.26.0, and is still on maintenance Support
(security fixes, no new features): runs until May 31, 2029.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

db/db_sqlite3.c

@@ -570,6 +570,7 @@ static bool complete_table_manip(struct db *db,
 	return true;
 }
 
+/* FIXME: sqlite3 version 3.25.0 (2018-09-15) supports ALTER TABLE RENAME */
 static bool db_sqlite3_rename_column(struct db *db,
 				     const char *tablename,
 				     const char *from, const char *to)

plugins/sql.c

@@ -873,8 +873,8 @@ static struct command_result *channels_refresh(struct command *cmd,
 
 			plugin_log(cmd->plugin, LOG_DBG, "Refreshing channel: %s",
 				   fmt_short_channel_id(tmpctx, scid));
-			/* FIXME: sqlite 3.24.0 (2018-06-04) added UPSERT, but
-			 * we don't require it. */
+			/* FIXME: sqlite3 version 3.24.0 (2018-06-04) added
+			 * UPSERT, but we don't require it. */
 			delete_channel_from_db(cmd, scid);
 			req = jsonrpc_request_start(cmd, "listchannels",
 						    listchannels_one_done,

wallet/db.c

@@ -1631,8 +1631,8 @@ static void migrate_channels_scids_as_integers(struct lightningd *ld,
 	/* FIXME: We cannot use ->delete_columns to remove
 	 * short_channel_id, as other tables reference the channels
 	 * (and sqlite3 has them referencing a now-deleted table!).
-	 * When we can assume sqlite3 2021-04-19 (3.35.5), we can
-	 * simply use DROP COLUMN (yay!) */
+	 * When we can assume sqlite3 version 3.35.5 (2021-04-19),
+	 * we can simply use DROP COLUMN (yay!) */
 
 	/* So null-out the unused column, at least! */
 	stmt = db_prepare_v2(db, SQL("UPDATE channels"