invoices: also run amount-validator on setter

- @amount_msat.validator prevents the creation of invoices with e.g. too large amounts
- however the qml gui is mutating invoices by directly setting the `amount_msat` field,
  and it looks like attrs validators only run during init.
  We can use `on_setattr` (introduced in attrs==20.1.0).
- a wallet db upgrade is added to rm existing insane invoices
- btw the qml gui was already doing its own input validation on the textedit
  (see qeconfig.btcAmountRegex). however that only limits the input to not have more
  chars than what is needed to represent 21M BTC (e.g. you can still enter 99M BTC,
  which the invoice logic does not tolerate later on - but is normally caught).

fixes https://github.com/spesmilo/electrum/issues/8582

electrum/invoices.py

@@ -94,13 +94,18 @@ class BaseInvoice(StoredObject):
     """
     Base class for Invoice and Request
     In the code, we use 'invoice' for outgoing payments, and 'request' for incoming payments.
+
+    TODO this class is getting too complicated for "attrs"... maybe we should rewrite it without.
     """
 
     # mandatory fields
-    amount_msat = attr.ib(kw_only=True)  # type: Optional[Union[int, str]]  # can be '!' or None
+    amount_msat = attr.ib(  # can be '!' or None
+        kw_only=True, on_setattr=attr.setters.validate)  # type: Optional[Union[int, str]]
     message = attr.ib(type=str, kw_only=True)
-    time = attr.ib(type=int, kw_only=True, validator=attr.validators.instance_of(int))  # timestamp of the invoice
-    exp = attr.ib(type=int, kw_only=True, validator=attr.validators.instance_of(int))   # expiration delay (relative). 0 means never
+    time = attr.ib(  # timestamp of the invoice
+        type=int, kw_only=True, validator=attr.validators.instance_of(int), on_setattr=attr.setters.validate)
+    exp = attr.ib(  # expiration delay (relative). 0 means never
+        type=int, kw_only=True, validator=attr.validators.instance_of(int), on_setattr=attr.setters.validate)
 
     # optional fields.
     # an request (incoming) can be satisfied onchain, using lightning or using a swap
@@ -108,7 +113,8 @@ class BaseInvoice(StoredObject):
 
     # onchain only
     outputs = attr.ib(kw_only=True, converter=_decode_outputs)  # type: Optional[List[PartialTxOutput]]
-    height = attr.ib(type=int, kw_only=True, validator=attr.validators.instance_of(int)) # only for receiving
+    height = attr.ib(  # only for receiving
+        type=int, kw_only=True, validator=attr.validators.instance_of(int), on_setattr=attr.setters.validate)
     bip70 = attr.ib(type=str, kw_only=True)  # type: Optional[str]
     #bip70_requestor = attr.ib(type=str, kw_only=True)  # type: Optional[str]