Fix gravity for areas in space near station (#88176)

## About The Pull Request
- Fixes #76827

This removes gravity from the following areas:
- Solars
- Nearstation
- Space
- Ordnance bomb testing

##### Note - Asteroid areas (Tram) or planet areas (Ice) are not
affected.

<details>
<summary>Examples of outside gravity turfs</summary>


![dreamseeker_Mml3HmRaIK](https://github.com/user-attachments/assets/b6e410b7-e242-4c77-8cc8-34e15bd96b42)

</details>

Now stepping on floor plating in those areas will result in mobs
drifting. (like they do inside the station when gravity is offline)
Stepping on or near catwalks, lattices, or walls stills controls your
movement. Also refactored a little bit of the `has_gravity` code to use
defines instead of `TRUE/FALSE`.

## Why It's Good For The Game
Consistency. Mag boots are highly recommended when performing EVA.

## Changelog
🆑
fix: Fix gravity for areas in space near station (solars, nearspace,
bomb testing, etc.)
/🆑

code/modules/mining/equipment/survival_pod.dm

@@ -4,7 +4,7 @@
 	icon_state = "away"
 	static_lighting = TRUE
 	requires_power = FALSE
-	has_gravity = STANDARD_GRAVITY
+	default_gravity = STANDARD_GRAVITY
 	area_flags = BLOBS_ALLOWED | UNIQUE_AREA
 	flags_1 = CAN_BE_DIRTY_1