Move PHP minimum to 7.4

7.3 isn't actively tested anymore, and since 7.4 has been out for so long already, it doesn't make sense to keep pandering to users who can't be bothered to update their PHP binaries.
In most cases, these users are also experiencing problems caused by older versions of pthreads that were long since fixed.
In addition, pthreads is no longer tested or supported on 7.3, so users can't get newer bug fixes even if they wanted to.

.github/workflows/main.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         image: [ubuntu-20.04]
-        php: [7.3.28, 7.4.20]
+        php: [7.4.20]
 
     steps:
       - uses: actions/checkout@v2 #needed for build.sh
@@ -37,7 +37,7 @@ jobs:
       fail-fast: false
       matrix:
         image: [ubuntu-20.04]
-        php: [7.3.28, 7.4.20]
+        php: [7.4.20]
 
     steps:
       - uses: actions/checkout@v2
@@ -87,7 +87,7 @@ jobs:
       fail-fast: false
       matrix:
         image: [ubuntu-20.04]
-        php: [7.3.28, 7.4.20]
+        php: [7.4.20]
 
     steps:
       - uses: actions/checkout@v2
@@ -137,7 +137,7 @@ jobs:
       fail-fast: false
       matrix:
         image: [ubuntu-20.04]
-        php: [7.3.28, 7.4.20]
+        php: [7.4.20]
 
     steps:
       - uses: actions/checkout@v2
@@ -189,7 +189,7 @@ jobs:
       fail-fast: false
       matrix:
         image: [ubuntu-20.04]
-        php: [7.3.28, 7.4.20]
+        php: [7.4.20]
 
     steps:
       - uses: actions/checkout@v2