Merge tag 'v1.26.0rc2' into develop

Synapse 1.26.0rc2 (2021-01-25)
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Bugfixes
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- Fix receipts and account data not being sent down sync. Introduced in v1.26.0rc1. ([\#9193](https://github.com/matrix-org/synapse/issues/9193), [\#9195](https://github.com/matrix-org/synapse/issues/9195))
- Fix chain cover update to handle events with duplicate auth events. Introduced in v1.26.0rc1. ([\#9210](https://github.com/matrix-org/synapse/issues/9210))

Internal Changes
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- Add an `oidc-` prefix to any `idp_id`s which are given in the `oidc_providers` configuration. ([\#9189](https://github.com/matrix-org/synapse/issues/9189))
- Bump minimum `psycopg2` version to v2.8. ([\#9204](https://github.com/matrix-org/synapse/issues/9204))