Bump macOS Xcode baseline to 11?

[Stephan Bergmann]

Our baseline on macOS is currently Xcode 9.3 (see 
<https://gerrit.libreoffice.org/plugins/gitiles/core/+/b4f666f2e677b05cab8395fe7972b45b15f60c3f%5E!/> 
"Bump Xcode baseline to 9.3").

However, I assume that most, if not all Mac that are used at least by 
TDF to do LO master builds are using current Xcode 11:

For one, I assume that 
<https://gerrit.libreoffice.org/plugins/gitiles/core/+/f67e5ef9a5c71f3b35b1c67eb72794e44cc15410%5E!/> 
"Drop broken filter-out of -bind_at_load for Executable_soffice_bin on 
macOS" would have failed with ld in Xcode < 11 not supporting 
-platform_version.  Yet, the corresponding Gerrit Jenkins build 
<https://ci.libreoffice.org/job/gerrit_mac/54097/> succeeded, and I got 
no reports about failed macOS builds at 
<https://tinderbox.libreoffice.org/MASTER/status.html> after the above 
commit and before 
<https://gerrit.libreoffice.org/plugins/gitiles/core/+/b7fd89100d8653dc73955780358fe31d38b68ebf%5E!/> 
"tdf#122218: Baseline Xcode 9.3 ld presumably doesn't support 
-platform_version".

For another, my question

> Feb 14 10:28:41 <sberg> cloph_away, all: do we have any macOS build machines that actually run the build baseline, Xcode 9.3 (and ideally macOS 10.13.2, but that's only a bonus)?

on #libreoffice-dev got no answer.

Now, as seen with 
<https://gerrit.libreoffice.org/plugins/gitiles/core/+/b7fd89100d8653dc73955780358fe31d38b68ebf%5E!/> 
"tdf#122218: Baseline Xcode 9.3 ld presumably doesn't support 
-platform_version" mentioned above, it would be useful if we could bump 
the macOS baseline to Xcode 11.

The current Xcode version appears to be 11.3.1.  I have no idea whether 
we would need/want to explicitly require Xcode 11.3 or be happy with 
just Xcode 11, but if we bump anyway it would probably make sense to 
bump to Xcode 11.3, just to be on the safe side.  (See the "whether 
Xcode is new enough" check in configure.ac.)

According to the Mac App Store, the minimum system requirement for the 
current Xcode 11.3.1 is "macOS 10.14.4 or later".  Unless prior versions 
of Xcode 11 would serve our needs too and had less demanding 
requirements, that would probably mean that we would need to bump our 
build baseline too, from 10.13.2 to 10.4.4.

(For iOS, our baseline is documented as Xcode 9.3, too.  I have no 
opinion whether or not we would want to bump that as well.  At least for 
the commit mentioned above it is not relevant, as that is macOS-only.)

Any opinions on bumping the macOS build baseline to Xcode 11.3 and macOS 
10.4.4?