Reasonable boost baseline?

[Stephan Bergmann]

The current baseline check

   AX_BOOST_BASE(1.47)

in configure is unchanged since 2013 
(<https://gerrit.libreoffice.org/plugins/gitiles/core/+diff/0d5b9f1ad4b132d8d0cc22af1f524f971c0166db%5E!> 
"check for boost >= 1.47 and SHA512_CTX (openssl >= 0.9.8) in 
configure"), and is likely no longer meaningful (in the sense that 
presumably nobody would try a build of recent LO with such an old Boost 
anyway).

The latest version offered at <https://www.boost.org/> is 1.72.0, and 
our bundled external/boost is 1.69.0 on libreoffice-6-3 and -6-4 and 
1.71.0 on master.

<https://gerrit.libreoffice.org/c/core/+/90770> "Fix build with boost < 
1.68" is a request to make libreoffice-6-4 build with older Boost, so 
there is apparently demand for that (though the Gerrit change lacks 
further information what that demand looks like exactly, i.e., what 
version of Boost, and why people would be stuck with that version).

There is also back-/forth-ports of that Gerrit change to libreoffice-6-3 
(<https://gerrit.libreoffice.org/c/core/+/90808>) and master 
(<https://gerrit.libreoffice.org/c/core/+/90807>), but it is unclear to 
me whether those are borne out of any actual demand, or were merely done 
"over-enthusiastically" in response to my question "Why would this only 
be necessary on libreoffice-6-4, but not on master?" 
(<https://gerrit.libreoffice.org/c/core/+/90770/4#message-1f2f934499721fea91e333ddc1f6e08bdfb766a1>)

I would like to start a discussion about the actual Boost baselines we 
need to support on libreoffice-6-3, -6-4, and master.  Lacking any 
communicated reason why a branch should support a specific lower 
baseline, I would suggest we stick to the version of our bundled 
external/boost on a branch as the baseline for that branch (and update 
the configure.ac AX_BOOST_BASE check accordingly).