x/tools/gopls: drop support for building at Go 1.13-15

[findleyr]

As described in our support documentation, gopls only supports building with the last four Go versions.

#50825 outlines a long-term plan to remove support for building gopls at legacy Go versions, but in the meantime we should drop support for Go 1.13 and Go 1.14 now that Go 1.18 is released. This issue tracks dropping support for 1.13.

Action items:

• note the end-of-life for Go 1.13 support in our v0.8.4 release notes
• check with the VS Code team to update our compatibility matrix
• document our new compatibility matrix.
• remove Kokoro 1.13 CI

CC @hyangah

[findleyr]

This is done.

As of https://go.dev/cl/457835, gopls no longer builds at Go versions before Go 1.16.