nomination-pools: add permissionless condition to chill (#3453)

Currently, pool member funds cannot be unbonded if the depositor's stake
is less than `MinNominatorBond`. This usually happens after
`T::MinNominatorBond` is increased.

To fix this, the above mentioned condition is added as a case for
permissionless dispatch of `chill`. After pool is chilled, pool members
can unbond their funds since pool won't be nominating anymore.

Consequently, same check is added to `nominate` call, i.e pool can not
start nominating if it's depositor does not have `MinNominatorBond`

cc @Ank4n @kianenigma 

closes #2350

Polkadot address: 16FqwPZ8GRC5U5D4Fu7W33nA55ZXzXGWHwmbnE1eT6pxuqcT

---------

Co-authored-by: Ankan <10196091+Ank4n@users.noreply.github.com>
Co-authored-by: Gonçalo Pestana <g6pestana@gmail.com>
Co-authored-by: command-bot <>

prdoc/pr_3453.prdoc

@@ -0,0 +1,13 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: "[pallet-nomination-pools]: `chill` is permissionless if depositor's stake is less than `min_nominator_bond`"
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Nomination pools currently have an issue whereby member funds cannot be unbonded if the depositor bonded amount is less than `MinNominatorBond`.
+      This PR makes the `chill` function permissionless if this condition is met.
+      Consequently, `nominate` function also checks for `depositor` to have at least `MinNominatorBond`, and returns `MinimumBondNotMet` error if not.
+crates:
+  - name: pallet-nomination-pools