qt6,qt76: provide "upgrade" path on systems where the port is no longer supported

[reneeotten]

Description

The initial Qt6 port (and updates to newer versions I committed) had the supported versions incorrectly listed. That issue was corrected a while ago, but there are users who had the qt6 port installed on a system that is no longer supported. So instead they should install the qt67 or qt64 (sub)ports.

To provide an "upgrade" path for these situations I added some logic to the Portfiles that will set replaced_by on systems that no longer support the versions provided by the qt6 or qt67 ports.

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS x.y
Xcode x.y / Command Line Tools x.y.z

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@MarcusCalhoun-Lopez for port qt6, qt67.

[reneeotten]

@jmroot would you mind taking a look at this PR and see if I missed anything; I think this should provide an "upgrade" path for systems where the qt6 or qt67 ports are no longer supported.

[jmroot]

Looks reasonable to me.