Remove `pallet-getter` usage from pallet-transaction-payment (#4970)

As per #3326, removes usage of the `pallet::getter` macro from the
`transaction-payment` pallet. The syntax `StorageItem::<T, I>::get()`
should be used instead.

Also, adds public functions for compatibility.

NOTE: The `Releases` enum has been made public to transition
`StorageVersion` from `pub(super) type` to `pub type`.

cc @muraca

polkadot address: 5GsLutpKjbzsbTphebs9Uy4YK6gTN47MAaz6njPktidjR5cp

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Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>