Ensure EncryptionKey is regenerated when changing secrets provider

[stack72]

Fixes: #5835

when rotating a key in the Azure KeyVault secrets provider, we had
the following error:

error: secrets (code=InvalidArgument): keyvault.BaseClient#Decrypt: Failure responding to request: StatusCode=400 -- Original Error: autorest/azure: Service returned an error. Status=400 Code="BadParameter" Message="The parameter is incorrect.\r\n"

This was because we were not regenerating the EncrytpionKey when
we were changing the secrets provider. Therefore, we now ensure
that this key is regenerated and we can successfully change the secrets provider

To test this, I was able to run the following:

az group create -l westus2 -n stack72-secret-provider

az keyvault create -l westus2 -n stack72kv10 --resource-group stack72-secret-provider
az keyvault create -l westus2 -n stack72kv20 --resource-group stack72-secret-provider

az keyvault key create --name pulumi-secret --vault-name stack72kv10
az keyvault key create --name pulumi-secret --vault-name stack72kv20

az keyvault set-policy --name stack72kv10 --object-id <my subscription id> --key-permissions decrypt get create delete list update import backup restore recover encrypt
az keyvault set-policy --name stack72kv20 --object-id <my subscription id> --key-permissions decrypt get create delete list update import backup restore recover encrypt

This set up a ResourceGroup with 2 KeyVaults and a secret in each

I could then initialize a stack:

pulumi stack init dev --secrets-provider="azurekeyvault://stack72kv10.vault.azure.net/keys/pulumi-secret"

I added a secret and ensured it was able to be decrypted:

pulumi config set MyDBRootPassword Password1234! --secret

pulumi config --show-secrets
KEY               VALUE
MyDBRootPassword  Password1234!

I then changed the secrets provider:

pulumi stack change-secrets-provider "azurekeyvault://stack72kv20.vault.azure.net/keys/pulumi-secret"

I then was able to decrypt the secrets:

pulumi config --show-secrets
KEY               VALUE
MyDBRootPassword  Password1234!