mod+GitHub: shorten PR template, bump golang to 1.15

The golang version always needs to be in sync between the go.mod, the PR
checklist and the installation instructions.

We also shorten the Pull Request template by removing everything that we
have a CI check for and just refer to those checks instead.

docs/release-notes/release-notes-0.14.0.md

@@ -10,8 +10,11 @@ added.
 # Code Health
 
 ## Code cleanup, refactor, typo fixes
+
 * [Unused error check 
-removed](https://github.com/lightningnetwork/lnd/pull/5537).
+  removed](https://github.com/lightningnetwork/lnd/pull/5537).
+* [Shorten Pull Request check list by referring to the CI checks that are 
+  in place](https://github.com/lightningnetwork/lnd/pull/5545)
 
 # Contributors (Alphabetical Order)
 ErikEk