Deprecate MD5 as a checksum algorithm

[jmcphers]

We currently use OpenSSL's MD5 algorithm to add content checksums in several places. Generally we use this wrapper:

https://github.com/rstudio/rsconnect/blob/e6a7248acfb76d81e2ffff0837c43c34c70bf959/R/utils.R#L218-L219

However, some systems, most notably RHEL, can be placed into a FIPS compliant mode. The version of OpenSSL on these systems does not include MD5 since it's not considered a secure hash algorithm. Consequently, enabling FIPS mode on these systems removes the content checksums and with them the ability to publish content to Connect.

We should deprecate MD5 as a hash algorithm for content checksums; SHA-256 would be a better choice.

(Internal note: see support ticket 36904)

[aronatkins]

(Internal note: also see support ticket 38544)

[aronatkins]

rsconnect shifted to openssl::md5 from digest::digest here (along with other changes): https://github.com/rstudio/rsconnect/pull/116

The digest::digest MD5 algorithm is not bound to the FIPS mode in OpenSSL.

[hadley]

Just a note from the linked PR that when we come to tackle this seriously, some MD5 uses can simply be ripped out in favour of assuming https.