exim: update to version 4.96.1

exim: update to version 4.96.1

This is a security release.

JH/01 Bug 2999: Fix a possible OOB write in the external authenticator, which
      could be triggered by externally-supplied input.  Found by Trend Micro.
      CVE-2023-42115

JH/02 Bug 3000: Fix a possible OOB write in the SPA authenticator, which could
      be triggered by externally-controlled input.  Found by Trend Micro.
      CVE-2023-42116

JH/03 Bug 3001: Fix a possible OOB read in the SPA authenticator, which could
      be triggered by externally-controlled input.  Found by Trend Micro.
      CVE-2023-42114

Signed-off-by: Daniel Golle <daniel@makrotopia.org>