automation: Update matrix-synapse Docker tag to v1.91.1

This MR contains the following updates:

Package	Update	Change
matrix-synapse	minor	1.88.0 -> 1.91.1

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Release Notes

matrix-org/synapse

v1.91.1

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Bugfixes

• Fix a performance regression introduced in Synapse 1.91.0 where event persistence would cause an excessive linear growth in CPU usage. (#​16220)

v1.91.0

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No significant changes since 1.91.0rc1.

v1.90.0

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No significant changes since 1.90.0rc1.

v1.89.0

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No significant changes since 1.89.0rc1.

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