crypto/tls: disable RSA-PSS in TLS 1.2

Most of the issues that led to the decision on #30055 were related to
incompatibility with or faulty support for RSA-PSS (#29831, #29779,
v1.5 signatures). RSA-PSS is required by TLS 1.3, but is also available
to be negotiated in TLS 1.2.

Altering TLS 1.2 behavior based on GODEBUG=tls13=1 feels surprising, so
just disable RSA-PSS entirely in TLS 1.2 until TLS 1.3 is on by default,
so breakage happens all at once.

Updates #30055

Change-Id: Iee90454a20ded8895e5302e8bcbcd32e4e3031c2
Reviewed-on: https://go-review.googlesource.com/c/160998
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>
16 files changed
tree: 2e0003c76862f5f80907778c5e6265fdb7c5814f
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. AUTHORS
  11. CONTRIBUTING.md
  12. CONTRIBUTORS
  13. favicon.ico
  14. LICENSE
  15. PATENTS
  16. README.md
  17. robots.txt
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