Fully deprecate TLS rehandshakes

Previously, the documentation indicated that it was possible to call
g_tls_connection_handshake() after an initial handshake to trigger a
rehandshake, but only if TLS 1.2 or older is in use. However, there is
no documented way to ensure TLS 1.2 gets used. Nowadays, TLS 1.3 is used
by default.

I'm removing support for rehandshaking from glib-networking, as part of
a large refactoring where keeping rehandshakes would have entailed
significant additional complexity. So let's update the documentation to
indicate this is no longer ever supported. Applications should not
notice any difference.

Also, sync some previous handshake and rehandshake changes from
GTlsConnection to GDtlsConnection that were missed by mistake. I
try to remember to always update GDtlsConnection when touching
GTlsConnection documentation, but it's easy to forget.
15 jobs for mcatanzaro/rehandshake in 22 minutes and 23 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Style Check
passed #547042
style-check-diff

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  Build
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cross-android_api21_arm64

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cross-android_api28_arm64

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cross-mingw64

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debian-stable-x86_64

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fedora-x86_64

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freebsd-11
freebsd-11-x86_64

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freebsd-12
freebsd-12-x86_64

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G_DISABLE_ASSERT

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installed-tests

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win32
msys2-mingw32

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vs2017-x64

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  Coverage
passed #547054
coverage

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78.1%
 
  Analysis
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scan-build

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failed #547055
allowed to fail
valgrind

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Name Stage Failure
failed
valgrind Analysis
Uploading artifacts...
_build/config.h: found 1 matching files
_build/glib/glibconfig.h: found 1 matching files

_build/meson-logs: found 5 matching files

Uploading artifacts to coordinator... ok
id=547055 responseStatus=201 Created token=nmmc8yFZ
Uploading artifacts...
WARNING: _build/valgrind-report.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1