Snap updates for 3.0 GA (dotnet/core-setup#8502)

* Snap updates for 3.0 GA

* Updates for  review comments


Commit migrated from https://github.com/dotnet/core-setup/commit/5491acd84b92034d5d53e226682f053a7b376ad9

src/installer/pkg/packaging/snaps/dotnet-runtime-2.1/snap/snapcraft.yaml

 name: dotnet-runtime-21
+base: core18
 version: 2.1.13
 summary: Cross-Platform .NET Core Runtime. 
 description: |
-  .NET Core 2.1 Runtime and ASP.NET Core. https://dot.net/core.
+  .NET Core runtimes and libraries which are optimized for running .NET Core apps in production. See https://dot.net/core.
+  .NET Core is a general purpose development platform maintained by Microsoft. 
 
 grade: stable
 confinement: strict
@@ -16,8 +18,6 @@ apps:
       - removable-media
       - home
 
-base: core18
-
 slots:
   dotnet-runtime:
     content: dotnet-runtime-21

src/installer/pkg/packaging/snaps/dotnet-runtime-2.2/snap/snapcraft.yaml

 name: dotnet-runtime-22
+base: core18
 version: 2.2.7
 summary: Cross-Platform .NET Core Runtime. 
 description: |
-  .NET Core 2.2 Runtime and ASP.NET Core. https://dot.net/core.
+  .NET Core runtimes and libraries which are optimized for running .NET Core apps in production. See https://dot.net/core.
+  .NET Core is a general purpose development platform maintained by Microsoft. 
 
 grade: stable
 confinement: strict
@@ -16,8 +18,6 @@ apps:
       - removable-media
       - home
 
-base: core18
-
 slots:
   dotnet-runtime:
     content: dotnet-runtime-22

src/installer/pkg/packaging/snaps/dotnet-runtime-3.0/snap/snapcraft.yaml

+name: dotnet-runtime-30
+base: core18
+version: 3.0.0
+summary: Cross-Platform .NET Core Runtime. 
+description: |
+  .NET Core runtimes and libraries which are optimized for running .NET Core apps in production. See https://dot.net/core.
+  .NET Core is a general purpose development platform maintained by Microsoft. 
+
+grade: stable
+confinement: strict
+
+apps:
+  dotnet:
+    command: dotnet
+    plugs:
+      - network
+      - network-bind
+      - removable-media
+      - home
+
+slots:
+  dotnet-runtime:
+    content: dotnet-runtime-30
+    interface: content
+    read: [/]
+
+parts:
+  dotnet-runtime:
+    plugin: dump
+    source: https://download.visualstudio.microsoft.com/download/pr/a5ff9cbb-d558-49d1-9fd2-410cb1c8b095/a940644f4133b81446cb3733a620983a/dotnet-runtime-3.0.0-linux-x64.tar.gz
+    source-checksum: sha512/0cabf85877eb3ee0415e6f8de9390c95ec90fa8f5a0fdb104f1163924fd52d89932a51c2e07b5c13a6b9802d5b6962676042a586ec8aff4f2a641d33c6c84dec
+    stage-packages:
+      - libicu60
+      - libssl1.0.0
+      - libcurl3
+      - libgssapi-krb5-2
+      - liblttng-ust0
+      - libstdc++6
+      - zlib1g
+      - libgcc1
+      - lldb
+      - libunwind8
+      - libtinfo5
+      - libdb5.3
+      - libc6
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